They probably don't care about that content.

My first guess would be that they snapshot the DOM in the JS tick immediately after window.onload completes. Maybe they have a short pause to let any fast timeouts or callbacks complete, but there's got to be a cutoff at some point (e.g. to stop an infinite wait for pages that continuously update a relative date). Of course, with their own JS engine, I bet they can get really fancy with the heuristics to determine when to take that snapshot.