SSRS Query Timeout settings: rsreportserver.config verses RDL Query.Timeout
The answer is in your first link:


  DatabaseQueryTimeout  -  Specifies the number of seconds after which a connection to the report server database times out.


This setting only applies to the connections from the RS services to the RS databases, not for individual reports to their data sources. This makes sense given:


  This value is passed to the System.Data.SQLClient.SQLCommand.CommandTimeout property


What if your data source is not SQL, what if it is Oracle or MongoDB or some other data source? This setting wouldn't exist for that connection object.

The answer to your question is, with no setting defined your timeout would be 30s.