Why would you use client-side JS for this though? It would be simple to subvert.

They likely do this:

1. User is expected to be served an ad video which lasts 45 seconds
2. User loads their preferred youtube channel up
3. Server marks a timestamp for this initial event
4. Server tracks if the ad has been downloaded AND if user tries to download the target video before the 45 seconds of the ad have elapsed
5. Wow amazing they can tell if you're blocking ads