Yeah, that's definitely true. Google sort of mapped an AWS concept onto its own cluster splits. However, there are enough regional-scale outages at all the major clouds that I don't personally place much stock in the idea of zones to begin with. The only way to get close to true 24/7 five-9's uptime with clouds is to be multi-region (and preferably multi-cloud).
I have experienced many outages that were contained to a specific availability zone in AWS, from power failures to flooding to cable cuts. You are correct that 5 9’s still requires multi-region though.
I think also Google as a whole has pretty good diversity. But Cloud customers demanded regions in big population centers and smaller countries where Google traditionally avoided due to cost reasons. This lead to less redundant sites that were often owned and/or operated by third parties. So in the US and Europe you can probably trust GCP zones quite literally. But other regions (I have heard lots of rumours in the APAC) they may not be quite as diverse as they appear.
I think most Googlers actually don't know the specifics (I certainly don't know), and if they could, they probably couldn't tell you. It's sort of common knowledge that some of them are like this, but not exactly which ones.