It's not that simple, coz validateAttachment() will return true for 2 different unconfirmed transactions that cancel the same order. After one of them is confirmed another transaction will never be confirmed. This makes it very cheap to DoS nodes - just send 1000000 transactions that cancel the same order. If we included all such transactions into blocks then DoSing would be very expensive.
Is there another way to counteract the DoS attack?
Yes, override Transaction.Type.isDuplicate() to check for such duplicate transactions, trying to cancel the same order, similar to the way it is already done for alias assignment transactions.