I don't understand. I think "for the time that he is in control" means "as long as he has > 50% hashing power". No need for 100%. As long as he has > 50% he can eventually overtake any fork that includes a new transaction.

In theory, given enough time (a very long time), someone able to maintain 50.0000001% of hashing power could eventually rewrite the entire blockchain after the genesis block.