Normally I think when people say "Redis isn't a DB", they mean "an update will always return successfully before the data has been committed to storage".
If your user makes a change, you record that in a Postgres database, you've informed the user of your success, and the power goes out on the Postgres database, then the user will not have been misled; the data is in the database. If you did it with a persistent Redis system, there is some reasonable probability that the user has been misled and their data is no longer available.
I don't think this is a good definition of a database. In reality, we take various decisions, some of which increase the likelihood data will be preserved across some kinds of failures, some of which decrease the likelihood. No one would say "you don't have a database because you take daily backups from a single, unreplicated Postgres instance". They say "you have made decisions which increase the possibility of dataloss under certain circumstances" (well, they say things I translate that way).