This is a valid explanation for the odd implementation of the Python protobuf library (the only convincing one I've heard). But, the last time I looked into it (just a couple of years ago), it didn't seem reasonably possible to make use of this.
I can't remember the exact issue I think maybe the C++ library would be statically linked into the Python C extension module, which made it virtually impossible to use it from your own C++ code. Or maybe the issue was just that there was no C++ version prebuilt on pypi (the default is the upb version).
Anyway, it seems a pity to go to such enormous lengths for one feature and then make it essentially unavailable.
But its kind of a clear example of a surprisingly complicated technical space. The big picture is that it's actually not weird when a simpler thing which has less constraints can be better on some axes users when compared to a complicated thing that delivers on many advanced constraints.
If "readability of the .py gencode instead of the .pyi gencode" was the worst pain point (or pet peeve) here then I actually suspect Buf wouldn't have even bothered, it's just one of these that is easy to see and explain.