What is this Matchmaking

If normals: People who don't normally play normals still have near starting mmrs, but are playing normals now since theres no ranked solo/duo and/or they don't care to play ranked in preseason. Hence potentially high ranked players with relatively low normals mmr will get matched with people at that low-ish mmr.

If ranked flex: Most people haven't really had time to climb ranks yet, and everyone was pulled down to a silverish starting mmr (along with some other mmr hiccups during the first weekish). So while the solo/duo ranks might be mixing challengers and silvers, they may all have within a tier or so of silverish flex mmr so as far as flex matchmaking goes, you're all close mmr and so it's 'fair'.

It'll probably be pretty rocky for the first couple months as the majority get roughly to where they 'belong'.