"If you want to get better, play with better players" is a myth designed to get more people to sign up for advanced and open so that the prize pool grows larger with dead money. Just my opinion.
Partly, though it's a myth that exists in other sports as well. Sports where people aren't playing for each other's entry fees.
There might be something to playing casual rounds with better players hastening your improvement. At least you have the chance to observe, discuss, and learn. Tournaments, tenuous at best, for all the reasons already given, and most particularly because it's only the first round where you're likely to play with better players.