That add-on is written in such a way that no input data or post data is ever modified. It simply adds inline CSS styles to the postbit template. For this reason, unless you used a different add-on or had modified it on your own, the default vBulletin editor behavior would typically be used.
I can only see it adding font, size, etc because of these settings if and only if the text was copied from an existing post, by highlighting the output HTML (not the editor) and then pasting that into the WYSIWYG editor for a wiki page. In general this will cause problems with any WYSIWYG solution, and is why many of them offer a button to remove formatting or paste without formatting.
However, I still agree that a tool to remove unwanted BB-Code tags from pages could be useful to make it easier for moderators to enforce specific coding standards for their wikis.