This characteristic you're mentioning is essential for your persistence database, but maybe not so much for a derived read-only databases (e.g. when using the CQRS pattern). Those guarantees and characteristics are often a trade off anyway.
Unfortunately, it's easier to think in terms of tools and substitutes. For instance, it's difficult for us to say "I used a combination of handwritten code and redis to provide some of the guarantees that I would've got for less handwritten code and Postgres". However, this is exactly that sort of article: it argues that sometimes, it is better to use Postgres even if the features you want a more naturally provided by Redis.
> And there I'm unsure what we consider essential characteristics of a database.
I doubt there really is anything. I don't think it's possible to actually narrow down "database" to some precise definition, so I think for instance the person who said "only an idiot thinks Redis is a database" needs to explain what they mean by "a database" before we can discuss the merits of their position.