To my understanding, the grid is managed to have a fixed number of cycles per day, which would make it perfectly accurate. A cheap crystal can easily lose a few seconds per day. And given how hard it is to keep 60 Hz noise out of circuits, I can’t imagine the sensing would be difficult.
The trick is that so many devices use DC power bricks now, because it outsources the UL-or-whatever compliance, and makes FCC easier too. It's much harder to sense 60Hz downstream of that.
