When banks use bitcoin instead of fiat, they could in fact create more 'bitcoin' in the same way.
You wouldn't own the bitcoin, just as now you don't own the real fiat. There would just be a number on your bank account that says how many bitcoin you own, but it's multiplied the same way as they do now.
It all comes down to the % of reserve they need to hold.
This might very well be what Revolut already does :D (you can 'purchase bitcoin' or sell it at the prices they determine; but you can't transfer it out of Revolut).
You wouldn't own the bitcoin, just as now you don't own the real fiat. There would just be a number on your bank account that says how many bitcoin you own, but it's multiplied the same way as they do now.
It all comes down to the % of reserve they need to hold.