The hypothesis is that the channel was only demonetized after a long period during which it generated ad revenue. Flipping the demonetization switch doesn't make that revenue disappear. Rather, that causes the revenue to stay in Google's pocket instead of being distributed to the channel owner.

The channel still gets the previous revenue leading up to the demonitization switch, so no, Google doesn't gain anything. In fact, they lose because they're now hosting videos that no longer generate revenue.