Tether is not even like a 'de facto "bank"'. Actual (modern, regulated) banks need to have all their liabilities (i.e. deposits) backed 100% with assets. This includes capital, liquidity assets like bonds and reserves, but mostly loans. Banks can and do originate money by issuing loans (creating a debt and a deposit in equal amounts), but if they don't have at least a decent chance of being repaid (and enough capital/insurance to cover defaults) then they are insolvent.
The difference with Tether seems to be that they appear to be creating the deposits (tether) without any evidence of actually having or creating enough assets to cover it all (either having money in the bank, or writing decent quality loans to back it).