It is one version of player agency, yes. There are others. To assert that this is the
only possible form of player agency would be incorrect.
And if it turns out that, ater being thrown together by whatever means are used, the characters can't, or don't, or won't get along well enough to function as a party then play it true: if they fight, they fight; if one or more leave or get tossed, let it happen; if they hang each other out to dry, so be it. Sooner or later they'll set a tone for themselves and largely only accept new characters who vaguely fit with that tone, and things will settle down. Leastwise, that's how it's happened every time I've seen it thus far.
Only if everyone at the table is agreed that such events are an acceptable part of the gameplay experience, and is of the specific mindset and approach such that repeated totally preventable setbacks and problems and (etc., etc.) are a good and desirable part of the experience.
This is not only far from universal, I would say it is quite uncommon in most gamers--not totally unheard-of, but certainly far from typical. Just as there are things you agree not to do because it would be crappy inappropriate behavior at the table, others have different standards for what is crappy inappropriate behavior at the table, and some of them see it as a perfectly good and worthwhile agreement to
skip over the "sooner or later" part and just
start from a group that has "set a tone for themselves" so that things don't
need to "settle down" but are in fact settled from the start, unless-and-until something beyond the pale occurs (and even that would probably be addressed, at least partially, through discussion between players (with or without the GM).
For a lot of gamers--I would argue the majority--whining "but it's what my
character would do!" when everyone glares at you because you did something that annoyed, frustrated, or upset the other players will get the perfectly appropriate response of, "Well then, because YOU are completely responsible for deciding what character to play, then
you chose to be an enormous cowpie, and we aren't really interested in dealing with your bovine feces." Blaming the character as though that somehow removes any responsibility from yourself is ridiculous--100% of the beliefs, thoughts, preferences, and choices belong to and come from you, the player of the character. "It's what my character would do!" acts like, because the character isn't identical to you, you're somehow completely innocent for any dickish things your character does, when...no, you're literally 100% responsible for those things.
"Don't be a dick" is generally a widespread basic minimum of human decency. If we add "to your friends", and then apply that to characters? "Don't be a dick to the other players" means...sure, you can play a character who
is just a big jerk in general, but their big jerk behavior better be mostly directed outward, or not so egregious as to invite a death glare from other players.
That, too, is a version of player agency--one that has begun from a different agreement between players, and thus has a different space of play.