Interpreting user agent strings is what amateur programmers do. I don't generally expect high standards from Google engineers (a whole other argument I won't entertain here) but that's still pretty tragic for a top-five website.

And besides, your claim is doubtful at best since Chromium Edge doesn't share any User Agent string elements with previous EdgeHTML versions. Your YT developers would have needed to be intentionally malicious to match "Edg/" as a trigger to downgrade the user experience.