> I didn't even need to bring up the DragonEgg cartel (Chandler?) going down the gcc-llvm-clang pathway used essentially for getting rid of the pesky GPL quoted above.

That's... not even close to what happened?

Historically, LLVM was at one point proposed by Chris Lattner, while he was at Apple, to be upstreamed into GCC (and relicensed to GPL, natch) for use as at the LTO optimization phase, which was declined. For most of its early existence, it used llvm-gcc as the frontend to generate LLVM IR. In the late '00s, serious effort was put into making a new frontend for LLVM IR which we know as clang, primarily by Apple at that point, which become self-hosting in 2009 or 2010. Basically the moment clang becomes self-hosting, everyone jumps ship from using llvm-gcc to using clang to make LLVM IR.

Google shows up around this time, I think primarily motivated by the possibility that Clang offered for mass rewriting capabilities, since it has extraordinarily good location tracking (compared to the other compilers available), which is necessary for good rewriting tools. The other major area of Google's focus at this time is actually MSVC compatibility, and I distinctly remember Chandler talking in one of his presentations that you need to be able to compile code to trust it well enough to rewrite your code, so I think the compatibility story here was mostly (again) for rewriting.

Also around this time, gcc gains proper plugin support, and llvm-gcc is reworked into dragonegg to take advantage of the proper plugin support. But because clang now exists, dragonegg is no longer very interesting, with almost all the residual attempts to use dragonegg essentially being limited to people trying to use it to get LLVM IR out of gfortran, as LLVM had no fully-working Fortran compiler at that point.

Again, that seems to be in no way demonstrating the pattern that was claimed to be happening often.

AFAIK Google did not take ownership of gcc, nor did they try to fork it without contributing to the original. They used GCC for a good couple of decades while contributing to it, but eventually switched to a different compiler. The same for clang, they neither "took it over" nor "forked it without trying to contribute".