Computers are general concrete state machines. Computer programs behave by conducting a computer into a specific state machine behaviour. So it can't be wrong to say that all computer programs describe state machines.

As others have said, I think a programming language must reduce to a state machine but I'm curious what your counter example is especially considering your conviction that it's not even close.

I think you’re correct practically speaking. The spirit of what you’re saying is true.

However technically, and pedantically, all programs must behave like state machines.

Btw it looks like it’s the author of the paper, not gpt4 making this confusing point so if one disagrees with the summary it’s a disagreement with the paper.

Here’s a link that covers your specific point: https://softwareengineering.stackexchange.com/a/288063