Saw it a few years ago and it gave me strong Hitchcock vibes. Agree that watching Sheen in a serious movie was a little weird, but the movie was decent

Sure, good point, I think of the movie Arrival as two parts:

For most of the movie, a scientist is struggling with a novel interesting scientific problem with guidance from subject matter experts who have established environmental knowledge but not theoretical insight, with a great deal of interference from funders, with inter-team rivalries and a collaborator / competitor tension with similar teams around the world. The problem in question is based on linguistics with the type of thoroughness that is never shown on screen and rarely in print SF. (Compare it to the "Shaka when the walls fell" episode of TNG. I like that episode! But it's cartoony by comparison.) So both the practice and the principle of the research shown has a scientific basis, and if the movie had ended with the lead scientist solving the problem then I think we'd all agree it's Hard SF. However, we also have the last part of the film.

You question the scientific plausibility of the last part of the film. Regarding the story the film is based on, apparently:

In the "Story Notes" section of Stories of Your Life and Others, Chiang writes that inspiration for "Story of Your Life" came from his fascination in the variational principle in physics. -source

but I don't know enough to judge that and though it was kind of uplifting, the last part of the film was qualitatively different from the first, and I agree seems a lot less "Hard SF".

To recap, I argue that at least the first part (a majority?) of the movie is Hard SF. Now the question is: does the last part disqualify it from a) being on this list and b) being Hard SF? Regarding a), the authors of the list say "Contact is hard sci-fi by association because it's not a very realistic film" so they are taking a very forgiving definition of Hard SF. Therefore I stand by my assertion that Arrival is qualified to be on that list. By virtue of the quality with which the first part of the movie proceeds, I argue that it also deserves to be on that list. Regarding b) whether Arrival is Hard SF beyond the definition used by that list I am less certain.

what’s the basis for

fiction

The Sapir-Whorf hypothesis, though decades old & sounding like it’s from Star Trek, is the basis, from actual linguists. Highly implausible for humans & long outdated, but as the film’s linguist consultant quips, “for aliens, all bets are off.”

https://en.m.wikipedia.org/wiki/Linguistic_relativity