He's the lead designer and writer. Because of that it seems he gets 100% of the following:
-if the person liked TSL, he gets all the praise for great writing
-if the person disliked TSL, he gets all the blame for poor writing
I liked the writing and I thought TSL's dialogue was deeper than KOTOR I, but regardless one thing people should keep in mind: Obsidian was allowed to write the story of TSL, but Lucasarts placed certain conditions on what had to be included and what couldn't be included. Perhaps the "True Sith" was a mandate from Lucasarts, and thus it became Obsidian's job to try and make it work in the story.