_lastWithdrawal[vault] will always be zero for new vaults, so the check is for 0 + minWithdrawalDelay which will always be less than block.timestamp
https://github.com/code-423n4/2022-04-backd/blob/c856714a50437cb33240a5964b63687c9876275b/backd/contracts/vault/VaultReserve.sol#L102-L103
