It has a matched play profile in the back of the book, it is just 0 pts cost. On warscroll builder you can also include it in your army.
It makes no sense that there is a restriction on having to place it if you can just opt not to. Either it is in your list or it isnt.
You're right that our battletome says that you must place the RE if the RE is part of your army. Except it is not.
FAQ says that terrains are not part of the army, and the matched play profile only means that you are allowed to set it up. RE cannot be in the army list.
It's another example of badly written rules. Still it remains the fact that terrains are not part of the army.
I cannot be forced to field something that is part of my army, if that something
is not a part of the army.
edit: IMO you are never forced to set up anything: pick for example the Chama skinks, if they are not placed at the beginning of the 4th round the unit is killed. You are not forced to set it up, you can decide to auto-kill your unit.
2nd edit: the sentence for the RE, says that it must be placed by the player before other terrains.
IMO that "must" doesn't mean that the player must place it, but that if you place it you must do it before the placement of all other terrains. The "must" just defines the timing of the placement, you cannot place it in a different way.
3rd edit (and i bet it won't be the last one):
IMO, the most (and probably the only) valid reasoning would be that:
1 - the above FAQ says "warscrolls are included for those instances when a
spell or
ability allows you to set up the terrain feature".
2 - If a terrain feature is fielded for a reason different than spells or abilitiies, the FAQ doesn't apply.
3 - the RE is not listed under "allegiance abilites", so the FAQ doesn't work in that case, and so the RE it's a part of the army, as written in the battletome.
4 - if all of the above reasoning is true, then the RE is, indeed, a part of your army. But in any case, this won't stop a seraphon opponent to use it: in all terrain warscrolls, "friendly unit" refers to the player who's actually controlling the objective.