... (Otherwise he could have pre-mined a few thousand blocks in advance, in secret, and use them later to steal other people's bitcoins.) ...
hhmmm? and how on Earth would he do that ? No: if it starts with the same genesis block, and is longer, that blockchain is bitcoin, by definition; and the blocks in the other (legitimate) branch are orphan blocks, ignored.
And even if you were right, the earthlings community would figure a way around this problem if it would pop up.
I was explaining why there is a newspaper quote in the genesis block. It was put there precisely to prevent this problem -- to prove to everybody that no one could have been pre-mining a secret chain before the genesis block was posted.