Wow, really....?
Your speculation.
Mine:
We don't know at what exact point in time the recording of the BoW rap happened. If it was near the time that Bryan had enough and left, it's possible that this was not communicated.
And even if it wasn't around that time...
Bryan says in the MJcast interview (as quoted before):
"And when I came in to
hear it a week or so later, it was their voice instead of mine. And I just assumed that meant ok, he just wanted to do it himself..."
In that situation (him just being meant to help out Bill to fix his rap, then leaving for some time, and then discovering that his rap was replaced), he doesn't seem like someone who then would have asked Michael "but did you hear my rap?".
And since Bryan only blames Bill for not being credited for the rap (quote: "it's also a story Michael is not reponsible for why I'm not credited there"), this indicates that the rap thing was all only between Bill and Bryan and Michael not knowing much about how it came to be.
Of course it's also possible that Michael did know about it all, and that Bill played Michael Bryans rap as well.