The database is an organized collection of data. Databases help in the storage and manipulation of data. Databases make data management easy. Let's discuss a few examples. Database Management System (DBMS) is a group of programs that enables the users to access databases, manipulate data, reporting/representation of data.
The Google Datastore and BigTable are two different things and Google datastore should not be confused with datastore data storage sense. Bigtable is more expensive than BigQuery. BigQuery does have proper joins and RDBMS like SQL language and it is cheaper, why not use big query. That being said, BigQuery does have some limitations, depending on the size of your data you might or might not encounter them.
Also, in terms of thinking in terms of datastore, I think the proper statement would have been "thinking in terms of NoSQL databases". There are too many of them available out there these days but when it comes to Google products except google cloud SQL (which is MySQL) everything else is NoSQL.