The "animation" excuse is complete bullshit obviously, but I do kinda get why they didn't want to make one of the co-op assassins female if they're all supposed to be the same guy (although I'm not sure why they're taking that route if they're marketing co-op as this game's gimmick but lolUbisoft).
Honestly the point is that they could have probably explained it very clearly in a way that wouldn't have been (as) annoying, but it seems like the idea that someone would
ask about why you can play as four white dudes and no other options... never crossed their mind. They didn't bother to have a logical correct answer ready because the idea that this would matter to anyone didn't occur to them. That's a pretty huge indicator about how important the idea of playable female characters
actually is to them, and the fact that they would lie and suggest that it totally would have happened if only they had time and money is a big part of what's so annoying.
It reminds me of the Tomb Raider PR a few years ago bringing up sexual assault imagery like that's a great selling point. It just suggests a lot about how the people talking view gender in the context of their game world, especially when it doesn't actually line up with the reality of the game's design.
Why bring up the animation issues at all if the characters aren't especially customizable in multiplayer and their appearance doesn't matter? Because apparently the first impulse was to pay lip-service to the idea of equality while actually not giving a shit or fully considering the implications.
The crowds use most simpler rigs than the main characters. Not only that. But imagine that you've been playing as Arno for the longest time, and suddenly you do a co-op mission and you're running in a feminine way because on another player's screen you're playing as a female. It just doesn't work. So no, they don't have a "vast majority of the animations for female characters."
What about the other secondary characters? Not a single woman there? No walking animations mocap'd AT ALL with women? Okay. Sure.
And I'm not even sure what you would consider running in a feminine way, so I'm not sure how to respond to that. But for example the characters in the multiplayer primarily all run the same way and the differences are mostly aesthetic because otherwise it would create an unbalanced multiplayer. Men and women, functionally, run the same in AC.