You are correct about the 40 second last reversible block time. The block confirmation time is not the same as irreversibility though. Each time a new block is confirmed counts as a 'confirmation' but one confirmation does not equal an irreversible transaction.

With BTC transactions for example, people typically say to wait at least three confirmations before trusting a transaction (sometimes more).

With STEEM, it is best to wait for at least 16 confirmations, since that is the point it reaches the irreversible block.