Actually you cannot query multiple databases by default there is db_set_active() which makes a call to the hardcoded db settings. But Drupal is not set up to handle multiple database natively.

To explain: Let's say you have entries of the type blog and guestbook. But you don't want to have them in the same table or database. You want to optimize as much as possible. You can not explicitly tell Drupal that blog entries should go in db_blogs.tbl_blogpost and that db_gbooks.tbl_gbpost should handle all nodes of the guestbook type. This would require rewriting the core system to allow it to see the different databases.

Now if this where another framework other than Drupal the system would be written so that you could first override the database handler as mentioned in the article using route.php. Then you would be able to write code that would do this on a need to have basis.

The article you mention is just reinforcing what is mentioned here. Scaling across multiple database servers and using clusters is not the same as using multiple databases.

As mentioned in the very first paragraphs of this article, there may be large enterprise sites using Drupal but they do not show how its done. My suspect nature tells me that they are simply using Drupal as a pretty front end to some very hardcore systems. Or perhaps they have hacked into the code so much to get everything to work that the CMS is only Drupal in name.

No you probabaly have not heard of any Joomla sites that are huge enterprise systems. But Joomla! does not propogandize that it is such a system. Joomla! pushes excellence in the area that suites it most.

The reason that this article exists is because one board of directors exec listened to all the propoganda surrounding Drupal and got mesmerized by the large brand names mentioning Drupal. He failed to do any research, one a code editor, ask and trust the opinions of the company programmers and made descisions based on inaccuracies. These decisions are now written in stone apparently. Because even after reading an article like this one the company will continue on trying to use Drupal and wonder "Why is it taking so long?" "Why are we spending more?".