Matt and Trey actually went to Obsidian, pitched the idea, and then went shopping for a publisher after they had already signed them up.
While this doesn't confirm that they chose the genre, my guess would be that they did given the context.
Has Obsidian commented? I saw that THQ had over 50 million in accounts payable on the balance sheet, I'd hate to see Obsidian not get payed for work already done.