"Finality" is just a fancy way of saying that non-blockchain entities have their own definition of when they consider a transaction irreversible. For example, if Alice sends to bittrex, then bittrex will consider the transaction irreversible after two confirmations, and credit her balance.

If the secret miners (attackers) don't get two blocks before the public miners get two blocks, then bittrex will credit Alice's balance and she will circumvent Hive account@dan's attempt at "censorship".

This is the part that Hive account@dan ignored when he made his faulty claims about being able to censor with 51% of the hashes.