The advatage might be they can ensure the compiler supports the new features qt wants to use.
reflection could replace moc, (likely c++29 needed) but if compilers don't implement that part of reflection qt can't use it. If qt can get compilers updated that helps them.
They routinely smoked GCC and Clang, and sometimes ARMs tools on a variety of tasks.
I'm not sure I see the advantage on Qt owning a compiler though - one of Qt's strengths is portability.