What's stopping him from doing a similar thing by selling deep OOTM options off of money he borrows on his margin account at any other broker?

Deep out of the money options have optionality whereas BitMex contracts are delta 1 products. They have literally nothing in common. The relevant difference in this case is that the option caps your upside.

My point is that risky financial plays are still possible even in the SEC regulated market. Maybe I misunderstood the parent poster though.

As per SEC requirements, brokers are required to quiz you to makes sure you understand the risks involved in trading derivatives.

Furthermore, selling naked options (Level 3 access) without hedging or verticals requires certain capital and margin requirements, I do not think it is easily possible to get 50x levered in options alone - you'd probably need /ES futures for that.