Makes sense to me. The kb players in the mixed queue are literally too busy to be paired up with kb only players.
They're not getting matched up with kb only players cause theyre already in games thanks to the much shorter queue times.
I'm sorry to say, the people who care the most about IBMM might just be a very vocal minority. Most people just want to get into a match as fast as possible and IBMM is off by default so they don't bother turning it on.
That's a direct result of the poorly implemented filters trying to band-aid the core issue, which is controller autoaim being so strong in the first place. People will always talk about a "vocal minority", as if the 'silent majority' would dislike or even notice what people closer to the heartbeat of the game would like to see. Greater effort should be taken naturally by matchmaking to pair players with their respective inputs, regardless of what filters are enabled.
I'm sure so many of the players you refer to indeed have no idea about this issue, and think they are getting out-BR'd in 9/10 engagements purely because the other player is a highly skilled ~20 year Halo veteran. Sometimes those players will actually be Halo veterans, but the AA further compounds with their game knowledge allowing them to stomp uncontested. Then there's another type of player who's not great, but in any given engagement they have a high chance of coming out on top against mouse players who actually have to aim. It's disheartening for them and I imagine a lot have simply quit playing because of it.
I played Halo back from CE in my Xbox days, but moved to PC a decade ago with Reach being the last time I ever plugged in a controller for an FPS. I still wanted the Halo 2 and 3 experience before MCC was even a thing, to play them as a true competitive PC experience. It arrived with MCC and feels like a complete waste of time all these years later. What was the point if this is how it's going to be? 343's initial decision has caused MCC PC to be an inexplicable home away from home for console players and pseudo-exploiters.