But anyway let me reiterate, if a enemy, or boss, have a skill and ability exclusive to a higher difficulty that they lack on lower difficulty or as an ally, but, as the enemy, they always have said skill (Such as Grima's dragon scale+++++ shit, I know, Grima is a bad example because you don't recruit him but I have vivid memories of his bullshit on lun+), then it's fair to list it as possibly/optional as it's a constant to said character depending how one plays the game.
RNG generated shit is aight to be exceptionally scrutinized, but like, if Aversa, Tiki, etc have a skill on a higher difficulty, but only on higher difficulty, but they always have it on the difficulty and isn't just a dice roll, well see above.
In the case of FEH though, them not having certain skills upon recruit is absolutely not reflective of what they canonically have, it's just another way to get you to spend due to the skill merging and sharing shit.
Is that sufficient?