How I got diamond membership for free
Chess.com has been gifting inactive players with free diamond memberships in an effort to bring old users back and increase membership. I know for sure that all members that went inactive 2-3+ years ago got memberships, so the same thing probably happened with you and everyone else that went inactive 6 or more months ago.
The same happened to me. 2 years ago, I couldn't watch this: https://www.chess.com/lessons/master-endgame
but know I can.
And I didn't even pay
Thanks chess.com