Except SBF didn't gamble away the money (still a criminal offence if he did); he spent it on lavish goods and apartments, with a lot of political donations. The very definition of fraud is deception, misappropriation, and loss to the users, all of which did happen.

In between placing a bet and returning the funds, the customer's funds were totally non-existent hence the loss to the user. That things later worked out after some years does not take away the misappropriation and that there was no actual money where it was meant to be at that moment, hence a fraud.