So, instead of using something lightweight and embeddable like QuickJS, they opted for Deno? Nothing specifically against it, just seems... overkill

See this comment:

There are other embeddable JS engines out there.

	▲	myvoiceismypass 3 days ago \| parent \| next [-]
		Hmm, like bun? Are there many others?
	▲	ohdeargodno 3 days ago \| parent \| prev [-]
		And you're welcome to implement them and submit them as a PR. The yt-dlp contributors are not in the business of writing or finding JS runtimes, they're trying to download videos.