Read the manga, Hagoromo's chakra is Six Path Senjutsu, by nature Indra's chakra is Senjutsu. Karin even compared it to CS level 2 chakra, which happens to be Senjutsu. It just doesn't have the NE influence that SM users possess.
The thing is we don't see anything. We know chidori is needed to channel the lightning, I might be mistaken but I'm pretty sure those where the hand signs for chidori before he puts his arm in the air. It is a bit ambiguous, but the fact that we never see him do any motions like that for any other jutsu lend weight to the theory. The only other jutsu I could think of that he could of used might be bringing down a chidori true spear(although that's only ever been that big in games, in the manga/anime it's a thinner blade type thing like he uses to save Karin from the hachibi)
