You're missing the entire point by what I mean by "effectively the same" and the point of my argument.

There are only ever two choices: your characters know each other beforehand, or the don't. Being forced to work together or working together by choice is irrelevant to what I'm talking about.

if the party is not planned together to be a cohesive group that are all guaranteed to have a motivation to play the written campaign AND have at least a reason to trust the party members, regardless of if they have personal history or not, is my method for avoiding the inevitable player who wants to bitch about not belong allowed to play their "edgy loner".

As I said before, even with literally using the threat of death forcing the character to work with the party, there is ALWAYS that one dipshit who wants to bitch and moan about how I'm "railroading them/preventing them from roleplaying their character" by doing so. Or, they waste time trying to argue for some loophole to go off and do their own thing, separate from the party yet somehow still "technically" doing the job. I am speaking from personal experience of over 10 years as a DM.

The other key thing about in media res is that you don't have that "inevitable round of introductions that feels like that time at the start of school when everyone had to stand up to say their name and one interesting fact about them". You're thrown into doing things before there's any chance for that. You get to know each other not beforehand, as in case 2, but as the adventure is going.

Yes, the characters are. The players, on the other hand, are all just sitting around a table rolling dice with no sense of urgency. They roll their dice, the encounter is over, and then the customary introductions start cause everyone is wondering what the other players have created for their character. Like, either you have been incredibly lucky with groups or have let Critical Roll give you rosey glasses about the role-play capabilities of the average player if you think doing things in media res makes a difference here.

I avoid all of this by just doing it in Session 0 with the afformentioned rules about character creation. It works. Ever since, I've never had to deal with it or any of the annoyances I have talked about.

Also, no, BG3 is not a good example. It is a video game that doesn't have to deal with fumbling IRL people who all have differing expectations and preferences. See, the biggest thing about the BG3 cast, is that the characters were all built in such a way so that they work together. Which is exactly what I have done with my method of character creation.

In media res will require players be cooperative enough to care to act, but it doesn't require they trust each other or know each other immediately. It definitely doesn't require pre-written specifically-designed characters.

See, the problem I have been talking about is that my method guarantees that players are cooperative enough to care to act that's the entire point of why I do it how I do it. Again, I am speaking from direct personal experience across 10+ years as a DM. Problem players will find a way to be a problem. So I nip it in the bud with a method that doesn't have to rely on the good-faith of the player, cause I've been burned by it more times than I can count.