It was easier than that. There was a premium for Bitcoin in China and South Korea (the "kimchi premium") for quite some time due to capital controls. People in those countries would buy Bitcoin domestically (driving up the price there), send them to exchanges abroad, and sell them there, thus having taken their money out.

If you wanted to run the arbitrage, you'd buy Bitcoin cheap abroad, send them to China or Korea, sell them expensive, and now you're stuck with fiat in China or Korea which you can't take out, either. BitMEX founder Arthur Hayes was closing the loop in the early days by frequently crossing the border from China to Hong Kong with a backpack full of cash. SBF and Alameda claimed to have made their early money similarly by finding a way to close the loop on the kimchi premium.