Actually, no.
> they are served from the same inscrutable urls as real video
This may be right, so host based adblocking may not work.
However, uBlock origin etc definitely block youtube ads and it works as a browser extension so they have access to the DOM and probably do something there.
I've noticed in the last few weeks that YouTube has some sort of fallback when an ad gets blocked. You get a white overlay with with impossible-to-read lightgrey text on it, with a blue button in the middle with a call to action on it (eg Signup Now, Click to Learn More). There is still a Skip Ad button and a widget to indicate when the ad will go away on its own.
I have two YouTube accounts, and at first it only happened on one. Now it is happening to both. Maybe it is something they are currently just testing.
This may be right, so host based adblocking may not work.
However, uBlock origin etc definitely block youtube ads and it works as a browser extension so they have access to the DOM and probably do something there.