The article you linked is about using a different appsetting for a connection string in your custom feature. it has no relation to the DatabaseHelper class.
For DatabaseHelper.GetReader, that first parameter is to override the connection string, if you want to connect to the mojoportal database you just pass in empty string, if you want to connect to another database you must pass in the full connection string not the name of a setting key.
and when using a datareader you must dispose of it so the only safe way is wrap it in a using so it gets disposed automatically
using(IDataReader reader = DatabaseHelper.GetReader("your real connection string", "SELECT * FROM [Products by Category]")))
if you feel to do that it will leave the connection open and exhaust the connection pool