From a moral perspective Dan wanting to relicense the project carries much more weight give his many years of contribution.
From a practical perspective the reason the rewrite went so well (significant performance boost, virtually no duplicate code) speaks to his skill and experience with the domain.
It's also a cause of friction here, because the fact that he knows the codebase so well makes him less credible as a clean-room implementer - his own biological neural weights are deeply biased by what he's learned from that existing code.