They are likely not solvent (i.e. they have enough liquid funds to cover everyone withdrawing their money all at once), but they apparently can cover a high percentage of USDT withdrawals. It is still very fishy b/c if you look at the lending rates on Bitfinex there is currently an 18.825% APR for USD. If there was really zero risk with USDT I think you'd see a whole lot more volume pumping through that system.