True, but if the bank says its worth 90k less, you still need to find that 90k. Unless I misread and they are reducing their mortgage offer amount by 10%.
If you are buying at what you thought was 80% LTV and the valuation is down 10%, as long as the bank is happy to fund at 88.8% LTV you don't need any more equity.
Not everyone is buying at 90%+ LTVs though, it's common for the constraint to be affordability / income multiples rather than deposit size.