So far I don't think anything in the manga has explicitly said that the So6P had mokuton, but I think we might be able to imply that he did from what we know about his story.
Here's what we know:
1. the So6P defeated the Juubi which was the progenitor of the world and the conglomeration of the bijuus' chakra.
2. the SO6P had the rinnegan which requires both uchiha and senju dna.
3. hashirama's mokuton techniques have been tied to his dna, which both Madara and Obito have used in order to get the Senju part of the Rinnegan equation.
4. mokuton has been explained to have the ability to supress the chakra of the bijuu.
So from these premises it seems likely (and or at least possible depending on how much you want to infer a power for the SO6P which aren't based on explicit manga feats as of yet) that the SO6P might have had mokuton in his arsenal when he fought the juubi... Does this kind of logic make sense to other people?
And if this kind of logic does make sense does that mean that Mokuton from Obito who looks on the verge of switching sides might be a critical part in the fight against the Juubi and his potential redemption?