I don't know for a fact how they manage it, but I bet the feedback is slow enough (those generators are heavy, and it probably takes a few cycles to even accurately measure a change) that they might not have to do much in the way of actively managing that feedback. The system will oscillate a bit (as it did in that video), but there's nothing inherently wrong with that, as long as it's within a certain set of limits.

(And really, the load changes are typically more gradual. Even "everyone just got home from work" is a fairly spread out event, compared to, say, the sudden loss of a few hundred MW of generating capacity...)