I would think the big factor for running yt-dlp in a browser would be CORS, not the language used to implement the downloader?

Yes, and other limitations of network access that Java applets did not have.

	▲	duskwuff 9 months ago \| parent [-]
		Java had restrictions on cross-domain access just like Javascript does and Flash did. Users could grant applets additional permissions - but that also granted them local permissions (like reading files) which were unambiguously a security risk.