If Kafka supported data processing (queries or whatever) then it would be much closer to databases. Also, databases are normally aware of the structure of the data (for example, columns).
Therefore, Kafka can hardly be viewed as a DBMS because it explicitly separates two major concerns:
* data management - how to represent data (Kafka)
* data processing - how to derive/infer new data (Kafka Streams - a separate library)
Theoretically, if they could combine these two layers of functionality in one system then it would be a database.