It's been extremely widely known that whoever created Bitcoin had a strong interest in Hashcash, and perhaps created that or worked on it, for years and years. If that's the only smoking gun, why didn't we identify Satoshi long ago?

You're right, "interested in Hashcash" describes dozens of people, and has been a known Satoshi filter for years.

The new claim is more specific: between 1997-1999, Back proposed combining Hashcash with b-money, adding inflation adjustment via increasing computational difficulty, and using hash trees for public timestamping.

That's most of Bitcoin's architecture in one package, a decade early.

The number of people who proposed that particular combination of ideas is much smaller than the number who were merely interested in Hashcash.