To be clear, do you think anything you said is in conflict with what I said?
The latest videos I've seen from Google still seem to focus on their high res maps being correlated with lidar and radar images to fine tune the GPS guidance. I still can't find much about how they deal with a world they didn't expect, like roadworks or a massive crash blocking the way.
When I hear GPS, I think of the satellite system and a receiver providing coordinates and time. Not one of those talking navigator units with built in maps etc, although they might be called simply GPS colloquially. Those things rely solely on GPS for localisation and often have outdated maps and crappy software. That is not how these cars work, although no doubt the cars have path planning algoritms and maps of course.
There really isn't anything "solely" about the use of the GPS [in google's self driving cars]. The cars can use the other sensors to precisely predict it's position and drive for long periods without GPS [they have to in order to be able to drive in tunnels and cities where GPS reception is not always available]. (If the other sensors alone are more precise than the GPS that in itself indicate they don't need the GPS.) I'm pretty sure they can drive entirely without GPS but they might have problem finding their initial location after a reboot without the GPS, if the car was moved from it's last known position while turned off, or something like that. I don't know enough details to speculate about that though.
Being able to read road signs, plan a new route if a road is blocked or handle a situation where the map doesn't agree with reality doesn't really have anything to do with GPS. I don't know how they handle that actually, but it's my impression they do not only rely on static data (in fact it's hinted at in that video as well, although indirectly). If you know of a source with details about that it would be much appreciated. Finding alternative routes is something all route planning software do though, can't imagine it would be a deal-breaker.