Mining on the hidden chain is not necessarily a head start. It would be if it was certain that this hidden chain will become the main chain. But if it doesn't, then mining on it was a waste, not a head start. Of course you don't know in advance, but that's exactly my point. If you don't know whether you're on the right track, you can't say you have a head start. And in the described situation, it's not guaranteed at all that the hidden chain will become the main chain.

The hidden chain can easily be discarded if the miner of the hidden chain doesn't find a 2nd block and if the miners on the public chain find a block and propagate it before the hidden chain is published. In that case, the public chain and hidden chains will be 2 competing heads, and other miners will decide which one wins. They will generally take the first block they saw, so most likely not the (previously) hidden chain. In that situation, mining on the hidden chain was a waste, not a head start. We could even say that the miners on the public chain had a head start. That's why I say there's no such thing as a head start.