Also a bit weird to do this between standard 1, 2 or 3's and Hoops or the other wildly different modes that take completely different skill sets to be good at. Those playlists should have different ranks and not just try and keep you even with the regular game.
You could be gold level in standard, but then be Grandmaster at Dropshot because it just clicks with you more.
A player’s predicted rank is based on their rank in other Competitive Playlists. While building this feature, we analyzed skill transference between playlists, and found that a player’s rank can be predicted with high accuracy using their ranks from other playlists.
It doesn't say that Diamond in 3s will make you Diamond in 1s, just that they can accurately predict where you should be in that mode.
For example, consider a player who was Silver in both Doubles and Hoops two Seasons ago, and now is Diamond in Doubles but hasn’t played Hoops since. When they begin their placement matches for Hoops, their rank may be automatically adjusted upwards.
Maybe not full ranks higher, but that is the intention and expressed in this example from the article that you might see a boost in MMR if you have higher ranks in other playlists.