The market was not jammed, only Robinhood was jammed. I could still buy GME on Schwab if I wanted
https://www.msn.com/en-us/money/personalfinance/gamestop-and...
"Some users are experiencing issues with trading platforms Vanguard, TD Ameritrade, and Charles Schwab due to heavy volume"
https://www.businessinsider.in/tech/news/some-users-are-expe...
Not to mention that the market is made up of all platforms and several of them were downright prohibiting GME buying. Some are listed in the article. Robinhood alone has more than 13 million users.
Apex is a large third-party clearinghouse used by M1 and others that restricted buys for all (most?) of its clients. TDA and Schwab both have their own internal clearinghouses so far as I know.