I'm guilty of using it as a DB, my home weather station writes to kafka topics, sometimes the postgres instance is down for months, no problems letting the kafka topic store the data until i get around to rebooting pg and restarting the connector.
The author presumes that every use case requires a transactional database. ACID is nice, especially if it's needed, but generally not needed, especially in many streaming data applications for which Kafka is most suitable.
The example they give is very simplistic. With the correct design of kafka topics and events the problem of the example can be fixed.
And according to oracle https://www.oracle.com/database/what-is-database/ :
> A database is an organized collection of structured information, or data, typically stored electronically in a computer system.
So Kafka clearly fits that definition.