If I have a pile of money, and add your money to my pile and go to the blackjack table put it all on black without your consent. That’s theft.
I'm guessing you're not very familiar with games of chance.
I definitely consider it akin to theft even if people don't quite want to use the word.
And importantly even if you ignore the fact that people aren't being made whole by the bankruptcy's 100% claim, people still won't have gotten their money back for years. So people are still significantly harmed despite the criminal winning some illicit bets.
So as you said the commingling was a large part of their crime.
But they did commit a bunch of straight up theft of customer funds to pay for marketing, political donations or stuff like houses for the employees.
The game you’re thinking of is roulette. Blackjack doesn’t have colors to bet on.
Putting it all on black is only theft if you lose. FedEx’s founder famously bet half his employee payroll on black and won.
FTX eventually lost.