I don't think churn is particularly high as a percentage of total workers. The company really is growing that fast. There have been over ten-thousand positions open via careers.google.com in the last several months (although I expect that to shrink in the coming days). And 10K isn't that unusual of a quarterly number.
No inside info here, just noting that that kind of hiring is par for the course.
Every engineer says that about their own products. My Amazon buddies all say AWS is a huge shitshow and my Microsoft buddies say Azure is huge mess. Technology is chaotic by nature