You're taking that too literally. A "multiverse" isn't a thing that really exists outside of stories, it's just a plot device used within certain stories. Yes, for the purposes of Crisis, all other DC screen productions are part of the Arrow-multiverse. But that in no way requires any other DC production to agree with that, because it's all just pretend for the sake of telling stories, and different storytellers can pretend different things. The 2003 CG-animated Spider-Man TV series on MTV was written as a direct sequel to the first Sam Raimi Spidey film, but the Raimi sequels totally ignored it (for one thing, it killed off Curt Connors in its first episode). Much as Superman Returns ignored every movie after Superman II, Terminator: Dark Fate ignored every movie after T2, Dallas ignored an entire season of itself, etc. Fiction is not binding. What one story says can be freely ignored by later stories, even those purporting to be in the same reality.
So it would be unrealistic to expect any other DC production to acknowledge the storytelling conceits of the Arrowverse, even in cases where the actors actually appeared in Crisis. There are countless examples of the same actor appearing as the same character in mutually incompatible continuities -- just off the top of my head, Don Adams reprised Maxwell Smart in at least two, probably three contradictory sequels to Get Smart (the feature film The Nude Bomb, the TV-movie revival, and the short-lived sequel sitcom). So, for instance, there'd be absolutely nothing to prevent Ezra Miller's Barry from saying in the Flashpoint movie that he's never met another incarnation of himself and got the idea for the name "Flash" in a totally different way than we saw here. It's all just pretend, after all. What the TV show pretended won't necessarily be what any other story pretends.
Of course they would. There have been countless examples throughout film and TV history of characters appearing on a different show or film without any implication that they shared a universe. Spock showing up on Carol Burnett. Lurch and Colonel Klink popping out of windows on Batman. The Penguin appearing in turn on The Monkees. Kevin McCarthy reprising his Invasion of the Body Snatchers character for a gag scene in the 1978 remake and again in Looney Tunes: Back in Action. Mulder & Scully appearing on The Simpsons. Detective Munch appearing everywhere.