It was just a time-based anchor. It does not prove the lack of a pre-mine. The proof of no premine was something any miner at the time could prove for themselves just by winning blocks.
How would it be possible to pre-mine if the genesis block has some newspaper headline in it that satoshi couldn't know in advance?
Without this text, he could have invented Bitcoin 10 years earlier and created 10 years worth of blocks (all without publishing), then created the whitepaper and a genesis block (that his 'private' chain is also based on) and let people mine. He would then have been able to attack the network as long as the chain was shorter than his private one by publishing it.
But due to this text in the genesis block, we can be fairly sure he didn't start a private chain before that newspaper article got published.