No way, man! They don't want to interfere with the viewing experience of their primary revenue source! (Ads) :-)

They are definitely not fetching video in the background.

Fetch from cold storage to thier CDN; whilst they fill your bandwidth with ads.

I WISH it would be prefetching video in background while showing ads.

But no, always goes to spinny wheel buffering after ad ends. Oh, and thats after having some spinny wheel to load the ad in first place ffs.