It’s also about a third thing:
capability!
KeychainAccess.app can do things Password.app can’t:
Certificate management with Certificate Assistant
Storing, exporting, managing certificates
secure notes for bank accounts, credit cards, software licenses, etc. (which I guess theoretically one could painstakingly move one-by-one to Notes.app as locked notes)
And then there’s the issue of cloud based things being convenient, but I still believe in the resilience of the master data being stored locally, and being available deep in the African bush, even without Starlink internet…