You are partly right.
To me, it looks like a strong majority of the best players got good while not in a clan, WM included. In fact, access to the best clans seems restricted to people who are already good, so your position seems counter-factural from the start.
when i joined WM 1 years ago, i was only decent/almost good (i was really bad on 1v1s, specialist of Eu map, decent/almost good on teamgames)
On past month i play a lot of tourney with WM-mate, and they help me to learn more, they help me to understand how pick/move/predict opponent.