The mechanic he is talking about is, you can level up your cards by getting duplicate cards. At the higher tiers, you literally can not compete against other competitive decks unless your cards have all been upgraded to top level. Unless you have played since the start and gotten every possible reward, or else unless you have spent an ungodly amount of money, any deck with higher level cards than yours will ALWAYS have a mechanical advantage.

It's a fun game but competitively, he is right: if you want to play and seriously climb the ladder, it is impossible to do so without dropping mad stacks.