Google are absolutely within the letter of the GPL, pedantically so. But maybe not the sprit.

We didn't have git tags when GPL was written in 1989, and while we did have sccs and rcs, (and early versions of cvs) they just weren't that widely used, and generally not used for distribution.

Even when GPL 3.0 was written in 2005-2007, source tarballs were still the primary form of distribution, even though it was starting to become standard to additionally provide anonymous cvs, svn, or one of the brand new distributed systems like git.

But these days git is the primary form of distribution, and source tarballs are noting more an afterthought. Hell, even tags are a bit of an afterthought on many projects. It's basically become the norm to expect an healthy revision history for any open source code.

Based on it's stated goals of "freedom to modify the software you use", IMO if the GPL was written (or updated) today, it would most likely require the distribution of revision history and restrict how much that history can be squashed/rewritten.