Most top movies last year [1] were shot outside of the US. Excluding animation movies, of course. I understand that the idea is “medium/long term” to move production to Hollywood area, but the short term impact can be massive.

[1] “Deadpool and Wolverine”, “Wicked” and “Beetlejuice Beetlejuice” in London, “Dune part 2” in Budapest and Italy, “Godzilla x Kong” in Australia. Only “Twisters” was filmed in the US.

The distinction here isn't US vs non-US movies, it's where they're produced. A lot of Hollywood output is produced outside the US now to escape the unions, to benefit from cheaper non-US workers and to benefit from tax breaks. These are American movies funded and written by Americans, but the bulk of the production staff aren't from there.

From the first paragraph of the article:

Donald Trump on Sunday announced on his Truth Social platform a 100% tariff on all movies “produced in Foreign Lands”, saying the US film industry was dying a “very fast death” due to the incentives that other countries were offering to draw American film-makers.

That is correct. Other countries are gutting Hollywood because Hollywood has become a hard place to make things. To pick a random example, the TV show Silo by Apple TV is made in the UK, not America, and the lead actress is Swedish. It's set in the USA, based on a story by an American author and produced by Apple but it's not made there.

This move is bad news for the UK and other countries that have built up a successful film industry but don't have the capital depth to fund big budget films, even with access to the US market. Now they lose access to US funding and can't easily export their films to the US either, assuming it goes through.