Read the paper. It claims that this is consistent with 2 spillover events, but it's also consistent with 1 spillover and an early mutation. The mutation between the 2 lineages could happen on either side of the spillover.

But a 2 spillover event suggests there was a pool of infected animals with multiple lineages that were all already capable of zoonosis. So why only a single secondary event? This suggests the pool was small, contact was limited, and that the pool wasn't sustained for long. OK, then it's comparably likely that a mutation would happen on either side of the spillover.