I assumed that most of the web based ones were using youtube-dl behind the scenes.

If you're downloading so you can watch it and not for rebroadcast there's literally no difference functionally or legally between clicking a link and opening it in a browser and using youtube-dl . The upside is when someone's account gets terminated for whatever reason you can still watch the content.

I read it as half-sarcastic.

A sincere half is that technical users can't quite understand what less-technical users would appreciate.

The tongue in cheek half of "why not just..." will always smell a bit like the infamous comment discussing how DropBox doesn't provide value, since anyone could set the same kindof thing up themselves.