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).
Central bank reserves aren’t the only assets that exist. Most countries already didn’t have a reserve requirement. CBRs are mostly required just for liquidity for interbank payments.
Banks that don’t have assets to cover all their liabilities (deposits) are insolvent. Non-delinquent loans are assets to the bank (liabilities to the customers) that are created when banks lend and create money (which become deposits - which are liabilities to the bank and assets to customers).
Only the central bank can originate money without creating debt.
Even the central bank can't originate money without creating debt. That's what money is, an exchangeable claim to resources, or assets. With base money, and the physical currency people hold in their wallet, the claim is notionally against the central bank's assets, that "back" the money and underly its value.
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).