> Isn't this textbook fair use?
Debatable. A good faith case can be made that its not.
> Nothing about these clips being shared on Twitter replaces the viewing experience of Cuties.
"Replacing the viewing experience" isn't necessary to be outside of fair use, and the work at issue is probably the trailer itself, not Cuties. Which matters a lot when you look at the "amount and substantiality" prong of fair use analysis.
> The tweets are definitely transformative and provide a critique of the film.
Sure, I don't think anyone would argue that the purpose prong of fair use is in dispute.
> Doesn't copyright law explicitly permit this use of copyrighted works?
There is very little explicit about the "fair use" defense; even how the four prongs of the test interact isn't explicit.