I can't speak for anyone but myself, obviously, but it seems that Tim Schafer and Double Fine do not want the game to labeled an RTS because that is simply not telling the entire story of what the game is. It does definitely have RTS elements, but it's juxtaposition of a bunch of genres. When something is labled an RTS, people immediately think of Starcraft or Command & Conquer; you have a bird's eye view of the battlefield, you manage your resources and troops, and there's a lot of micro-management.
Brutal Legend borrows some of those elements, since you do order the general direction of your troops, control unit production, and maintain control of fan geysers. However, most of your time is spent actually battling with your avatar. RTS implies that the gameplay is not very involved or personal, and that's not the case.
It's an action-strategy game or something.