Yes fractional reserve banking is a real thing, but real banks must publish audited financial statements so that outside observers can asses the banks financial position. From small local banks to national reserve banks, the financial statements are out in the open for depositors and borrowers to scrutinize. This isn't possible with Tether because the fiat side of their books are closed.
If everything is above board at Tether why not release the books? It's such an obvious PR win and would increase adoption of their currency.
They haven't done so because they know the books are cooked.