I disagree, but with nuance. I consider myself a skilled programmer, and I have switched languages many times throughout my career, and you are correct, it is not much of a big deal. There have also been times in my career where I've been responsible for whole swaths of an app from back to front end where the backend was in Java and the front was in HTML/CSS/"old school" JS (this was back in the JSP/server-side days). And yes, in that case it was not hard to switch back and forth.

The problem, though, is that, as you point out very well, the back and front ends have very different requirements, and so in most organizations you have separate teams focused on the front end and back end. If I'm a front end dev, I'm going to be spending day-in day-out working in Javascript and my JS ecosystem. It's not that it's hard for me to, for example, learn and use Python, the issue is that since I'm not "practicing" it every day it definitely gets rusty, and as time moves on the ecosystem moves on such that my environment likely needs some tweaking (venv vs. pipenv vs. virtualenv vs. conda - honestly it's been a while so I have no idea what is in widest use these days).

So if I'm on the front end, and I need say one extra piece of data that I know is stored in the DB exposed in some specific API call, for what is likely a trivial change for a backend dev, well now the time it takes for me to get my backend env in sync (again, which maybe I haven't bothered with in 3 months), refresh my knowledge of Python, work through any dependency update issues etc. I may have can often be longer than the time it takes to make the simple code change. So what usually happens is the front end dev just doesn't bother, they submit a ticket for the backend dev to do it and wait to sync until the next sprint.

Compare this with what happens when front and back end are both in TS - the front end dev usually has a short chat with the backend dev to make sure the changes will be OK, but since everything is immediately familiar can go and make the change just as quickly as the backend dev, without needing to wait for the backend dev to have time to do it.

I have seen this cultural change in depth over the past couple years, and it is just a pretty shocking change if you've ever experienced it if you've been used to teams that were more "siloed" between front and back ends.