What is this matchmaking?

this is normal the unranked players have to play with someone and being put against bronze 5 might not be their actual skill level it is called placement matches for a reason you are put against different teams of different ranks to see where you stand

Somewhere in mid-low silver is the 'starting' ranked mmr, so everywhere within a couple 100 mmr points of there is a hot mess of people still trying to find their 'proper' place, as you get the bronze 5 skill levels doing their initial dive, and the smurfs doing their initial climb, and everyone in between. You'll occasionally find people on the 'wrong' side of starting mmr too since in the first couple dozen games or so it's easy for ones actual skill level to be lost in the noise of random teams.

(As a very rough rule of thumb, it takes most people ~100 games to find their actual level. People will continue playing from there and continue to rise/fall of course, but MMR is roughly 'caught up' at that point and continuing to play/climb is more a side effect of your own skills increasing alongside your mmr, rather than your mmr just lagging behind your skill in the first place.)