It is very consistent. Distances are measured that way in AoS across the board. For example the range of spells, the range of shooting attacks, determining units control over objectives, the eligibility of declaring charges. Lastly, being in combat does not specify the requirement of line of sight. The only time vertical distance is measured in AoS is when assessing move distances for units without the Fly keyword.
It is inconsistent as getting into combat involves
moving over the barricade. The whole point of being stuck in combat is that the enemy has a zone of control you can't just enter or exit without risk cuz the other guy might follow you and stab you as you do so. If he has to climb over a barricade to get to you, his zone of control should be significantly diminished. A model's zone of control can basicly be seen as the area it can viably control by
moving into it at a moments notice. Hence the pile-in mechanic allowing just that, for the model to move to reach an opponent within his zone of control. It's not just an imaginary bubble that ignores things like (impassable) terrain.
As for attacks, spells etc. None of those involve the model
moving over the barricade, they just involve reaching over it with whatever attack you have. So as long as you are tall enough to concievably reach over it and stab/cut/shoot at an exposed bit of opponent you can viably hit him.
As for being stuck in the control zone not specifying line of sight… I mean yeah, technicly. However, this logic would have the increadibly stupid implication that you can put a wall between 2 units that's 50" tall, 0.5" wide and a 100" long and they're now in combat despite them having no concievable way of even knowing where the other guy is, let alone being able to ever reach them. Which is well, really stupid…
Also, I just read the attack passage in the general rulebook again. Melee attacks apparently don't technicly require line of sight, only the passage for ranged attacks
explicitly mentioned the need for line of sight. So in my previous example with the essentially infinitly tall and long wall as it covers the entire battlefield the two units could attack eachother
through the wall despite not being able to ever actually stand next to eachother. So RAW that's a thing apparently… a really really stupid thing…
Also needless to say, the range for being in control of objectives I would find similarly stupid since then you can control them through walls as well..
At least for stuff like spells and aura's and such you can claim some magical explenation, or the fact that shouting a command at a troops on the other side of a wall still works, especially seeing as most walls are decrepit ruines with holes in em.