Rate-limiting is still easy with IPv6, you just limit whole subnets (e.g. a /64) instead of single IP addresses. I guess Google has figured that out as well.

I actually do believe that this is real traffic. Here in Germany the number of connections with native IPv6 is rising constantly. At home all my devices are always using IPv6 when connecting to Google services because I also have native IPv6. Those 12.26% for Germany look pretty believable.