He isn't the only kb/m player and playing in a vaccuum though. He would lose to the kb/m players.
I am saying all things being equal, equal skill, you are giving one person a more precise and faster control scheme and one player has a handicap and you're saying it won't matter when consistently, everytime we have ever seen PC vs controller players especially in FPS, the keyboard/mouse players always destroy controller players.
So it isn't about a diamond 6 kb/m player being the same skill and rank as a diamond 6 controller player, one is always going to have an advantage, whether it is utilized as effectively depends on the player.
Simply put, the rank you land at is based around the principle that people you share a rank with have a similar likelihood to win as you. This fact doesn't change with additional input methods.
Yes a diamond kb/m players has an inherent aim advantage over controller users of the same rank. BUT the fact that they share a rank, means that they have an equal chance of winning... So either the kb/m player has a deficiency elsewhere, or the controller player is so good in other areas, that he can compensate for his aim disadvantage . Either way, it would be a close match between worthy opponents, which is the goal of a MM system.
The games that have tried to mix PC and console in the past did nothing to attempt to match opponents based on who they've demonstrated an ability to realistically compete against
@funk
With the above in mind, playing people above your rank would be no more or less problematic than it is now (except the fact that a higher population would make it less likely to happen). If I'm a plat 6 and I'm playing against a d4, Regardless of his input method, his likelihood of winning is the same as everyone else at d4.. If his chances of winning were higher, he'd have a higher rank.
At the end of the day, most kb/m players would rank well above contention with most controller players.