Ok, well I didn’t hear it when I was there but on the video it definitely sounds to me like the unbroken “ooooo” of a boo, if it’s a “yooooo”I can’t hear the associated “‘ull”. It also sounds lower pitched like a boo not the higher pitch of yoo ull.The "You" part does, and as you'll know, a lot of the time that part is elongated for as long as possible, so until you hear the 'Ull part, it sounds a lot like booing.
anyway I’m probably over thinking this on a Friday night…