LynneAndChad:
I would imagine that your WP installation would need to know nothing about primary.com and only have URLs and config options that reference addon.com or http://www.addon.com (decide www- or non-www first).
Also, that second Bluehost support article (#134) seems to try to attack a few needs within 1 .htaccess file. It tries to tackle the "non-primary.com" issue you want to solve, but also integrates the "non-www" solution as well. And also it seems like that solution only address URLs of when the visitor is attempting to access your homepage only -- but doesn't handle other pages on your addon domain.
I haven't tested this, but maybe give it a whirl:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.addondomain\.com
RewriteRule ^(.*)$ http://www.addondomain.com/$1 [R=301,L]
It means "if the hostname doesn't start with http://www.addondomain.com then replace the whole thing with http://www.addondomain.com/everything-else... (Addmittedly, however, I still haven't grasped the full extent of the $1 parameter, but I think that's how it works here.)
Good luck,
-WC