It depends what static refers to. In this case, it refers to static file creation without PHP (or Python) on the server. The content itself can stay static, JavaScript and its dynamic nature is just used to encrypt and decrypt.

It’s been always the same. Backend, server logic, database… dynamic content; on one hand. JavaScript runs on the browser for almost anything, sometimes for dynamic content. But it’s not tied. You could have an in-browser button with a counter for the numbers of times it was pressed (that’s actually an entry-level programmer exercise) and that’s a static site. If you saved the counter value to the server (e.g. database) then it’s not static anymore.

I think they mean taking a static site, and encrypting that with an attached JS. It could be done with a browser extension too (probably), but that way it would be less portable

No, static sites are usually more js heavy because they offset all the processing to the client.

This is one reason I avoid static sites, and just heavily cache the server.