There are two conditions for the chain obtained from header presynchronization:
- To minimize chain work
- The work must be done within 1 day of the active chain tip.
When you first start a Bitcoin Core node, the active chain tip is in the genesis block. There is a huge amount of accumulated work in the chain we pre-synchronized. So the first condition passes, but how does the second condition pass? This will be a lot more work than a day’s worth of genesis block work.