It's still fun that it's there, but it's not as big a deal as it sounds from the tweet.
That said I think a decent Python-native JS interpreter isn't that bad of an idea, it definitely needs a separate project and a more sophisticated architecture but it's an attainable goal.
This interpreter is built around matching specific Regex patterns and then immediately executing hard-coded behaviors with a few slots for parameters. It's missing a whole lot of the skeletal structure that would be necessary to "scale" it up to support a generally useful subset of JS, much less the entire language. Without the necessary structure, it would be a buggy mess that's impossible to maintain, and you can't just take the existing code and structure it better: it's built on the wrong foundation. That's what I mean when I say it won't scale.
That's not a judgement against the team that wrote it! It meets their needs fine, and choosing a minimal solution is great engineering that takes a lot of guts in the current software culture. I like what they've done here. Just don't take it and try to scale it up into a full JS runtime.
The specific site modules in youtube-dl will take care to extract the bare minimum necessary to solve whatever challenge.
but then this could be turned into a commandline browser that is able to interpret a whole web-page and save the resulting html structure instead of the source as curl/wget would do.
I bet someone's already started a YouTube downloader that uses a headless browser