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.
Also everyone on Unitymedia. Dual stack if you're a lucky older customer, DS lite (CPE has routable IPv6 and NATed IPv4) if you are new or get something changed.
I just noticed that a few weeks ago, with no change in contract. I have a resold Telekom VDSL line, and I was quite pleased to find that I now have a native IPv6 connection with a /56 assigned.
I'd tried some IPv6 settings in my router last year, and 6to4 worked pretty well at that point.
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.