The language has everything to do with it, because the language is the locus of practice. Rust practice is whatever it is, and is apparently pretty free with the use of "then" as a method name, which is fine. Javascript practice isn't the same as Rust practice, and Javascript practice includes a pretty strong norm around methods named "then".

That's why the next time I run into such a method, that doesn't belong to a promise and behave the way a promise's "then" method does, will be the first time I can remember, despite having worked primarily or exclusively in Javascript since well before promises even existed.

I'm sure there is an example somewhere on NPM of a wildcat "then", and that if you waste enough of your time you can find it. So what, though? People violate Rust idioms too from time to time, I'm sure. I doubt you'd argue that that calls Rust idioms themselves into question. Why does it do so with Javascript?