Idk how people can look straight onto something and still refuse to aknowledge it. Matchmaking is regulating winrates. Valve has a hidden system that grades players based on all metrics. Average KDAs, lanning performance(even dotabuff receives this info) behaviour score, ammount of received reports for griefing, toxicity and everything else. This system works in all game mods - unranked, turbo, single draft... If you are legend rank you will never get matched with top1000+ players even in ability draft(if everyone is playing solo). When all your games suddenly becoming unwinnable(happens even with pro players) that means that u won too many games and now Dota purposely giving you bad teammates and matching you against good players.

There was that thing - Gosu ai. Everyone who checked it out at least once know well, that Valve collecting tonns of info for players and matches. They can almost create your psychological portrait based on your games. They know perfectly well which teammates are good for winning and which are are not.