Your point stands, but one nit-pick:

The Boehm GC is only so it doesn't run out of RAM while bootstrapping, it uses the Ravenbrook MPS precise garbage collector once it's bootstrapped.

Also note that Christian reported that Ravenbrook was very responsive when he discovered a few deficiencies in the MPS library.

Also he used clang to walk his C++ code to automatically generate information in order to enable precise GC of C++ objects.