What does UI or UX have to do with language choice?

From what I can tell they use ASPX on their backend for the most part. So assuming that typescript is mostly used for frontend wasn't that far of a stretch.

Good UIs usually undergo rapid and extreme changes over very short periods of time. The choice of language could make it difficult or time consuming.

I've seen some great UIs that are cobbled together from bootstrap + several extensions with a bunch of monkey-patched CSS and other jQuery extensions... the code is a hodge-podge mess that I didn't enjoy working with, but the UI itself was quite pretty/useful.

I've seen the opposite as well. One does not beget the other. On my current project, I'm often doing a followup pass on the UI/UX for this project.

It doesn't in this case.

Sure it does. We often have designers jumping in and programming UI code themselves. Teaching them all about type systems would be very time consuming.

As consumers of a library? Quick, what kind of object can I use this function with?

function computeAge(birthday: Date): int;

I might have agreed once, but PayPal's UI/UX has been dramatically improving in the last year or so.