Not ragging on the guy as a player at all - as I said, he must be very good to have gone 5-0 at all. My point is that (at least at the time of this tournament) other Seraphon players were leaning into Salamanders and Bastiladons for a reason, and then along comes this guy with an off-meta KC list with just 4 Saurus units to clean the tables.
Just took a second look at his build, and I'm sure he built the lists very deliberately to make sure he had the tools he needed to win. But myself as a Basti / Skink purist just looks at this list wondering what the hell the secret sauce was. Even without any Bastis, Thunder Lizard looks like it would be a better pick for this list for Prime Warbeast, the extra wounds on monsters, and double-tapping the EotG. In my experience that outweighs the benefit of a +1 to hit on a bunch of units which frankly won't be that killy anyway in the Age of Savestacking due to lack of rend.
Didn't mean to come across as though I'm talking down on the player - I'm genuinely in awe.
Never took it that way, apologies if it came off like i did. Meme lists to me are just lists that are played for a laugh and this wasn't that.
To try and answer a couple of your questions, saurus knights actually hit really hard and like any seraphon list, its just threat overload. Carno, prime, blocks of knights, engine... its a lot of damage being delt in a lot of phases, and there's a good chunk of it that's just mortal wounds. Players often aren't taking prime warbeast even in thunder lizard (arcane might is the go to for the reroll on the slann) and not needing to spend a command ability to get +1 hit and then having a ton of access to it (+1 when charging, koatls CA, AoA) can be a huge help in getting reliable output. Part of the reason mawkrushas are so good is because they get triple the benefit from CA's, and the Koatls ability works in a similarish fashion.
Ultimately, i think players lean into salamanders/bastis/skinks because it's obvious and easier to get maximum output out of. A list like this plays really well into a lot of whats good in the meta right now, and a player like Sam is talented enough to maximize that benefit.
Like a lot of tournament play, matchups and battleplans will play a huge role, but it speaks to the player AND the list design that he was able to beat out a thunder lizard list in round 4 and a warclans list in round 5 to take the win at Everwinter.
Basically, I think our community looks at seraphon with a far to narrow lens. The book is disgustingly deep, amazingly efficient, and incredibly flexible. It gives us so many options to combat the meta, or even play some stuff you like and still perform admirably with it.
That may have been the first time koatls went 5-0 but it also has several 4-1 finishes. The "secret sauce" is it still leverages all the things that make seraphon good. A slann to control the magic phase, ranged mortals to threaten heroes (rse, comets, prime, engine), monsters to score battle tactics, good battleline and plentiful screens.
2 still isn't exactly a massive difference when the list that someone linked in another topic a while back had 10 dragons. The new list will still be 90% dragons.
Don't get me wrong it might be enough to kick it down a notch. It might make it so the list no longer dominates tournaments. And the whole "battleline if" definitly shouldn't be as widespread as it currently is, but it doesn't actually prevent spamming. There will still be the occasional player showing up with 8 dragons and ruin the fun for others cuz they now have to deal with that annoying nonsense, even if it is no longer meta. Which still isn't great.
2 dragons is literally a 20% reduction from 10 dragons. That's not insignificant.
Also, spam is inherently problematic, to an extend at least, once you spam powerfull units.
The main issue at the core of this is that a spam list forces the opponent to spam the appropriate counter(s) in return. This is fine if it's a relativly weak unit being spammed for which your opponent has many viable counters. But in the case of powerfull units the amount of viable counters are limited. Resulting in the opponent now also being forced to spam that particular counter.
Which isn't exactly great.
Spam is only a problem when you're able to spam a unit that's superior to all others in every slot, as you mentioned. But that's a problem with the unit, not with spam. It's actually why you're seeing the best players bring dragon lists that aren't just spamming dragons... because a vast majority of the time "spam" lists actually aren't the best. Just the easiest and most obvious to build.
That unit is still too strong whether you spam it or not. Preventing spam doesnt solve the problem of that unit still being too strong.
A great example of this was an article on the website a while back, where some guy explained his eel-spam list. And it pretty much boiled down to "the first unit or two of eels will probably be killed without achieving much, but then you still got X units of eels left and that should be more than enough to remove the main threats your opponent has. At which point you basicly have free reign, cuz you still have X eels while your opponent is screwed cuz you've now destroyed all of his potential counters if you did things right." Essentially this list relied on the fact that no opponent is going to bring enough counters against the eels to take it down before you killed all of his options.
Which isn't exactly fun for the opponent (very fun for that guy to figure out this tactic though

)
I've actually played against morton, great player. Eel spam was always a problem with eels and not spam. The unit was too strong, so it was spammed. It also falls into the "battleline if..." problem
If eels weren't battleline or weren't head and shoulders the best unit in the book, they wouldn't have been spammed.
Neither of those solutions require caps on spam. Because the problem was never spam. It was eels, and then the rest of the book being trash. You actually saw this problem being solved when they buffed turtles and sharks.
You can see a better example of this actually in seraphon. Salamanders are the best unit in the book, but you are rarely spamming them at the cost of everything... because other units actually have a role to play, especially outside of dracs tail. Even sally spam in dracs tail was a relatively uncommon build, and almost always included a bastiladon or allied help.