The question is from where did he get Senjutsu chakra, was it from Hagoromo(does having Hagoromo's chakra =Senjutsu chakra), or was it Hashirama's chakra he gave to him, or was it Kabuto or even the Juugo cells he had finally activated. Or (my favorite one) is that he already had the innate ability to passively absorb Nature's chakra from birth, but that ability was not awakened, and Oro suspecting that(He might have reached to this conclusion after years of researching about the Uchiha clan, he may have deduced that one Uchia will awaken this ability) gave Sasuke the curse seal of heaven to test if he's compatible with senjutsu chakra and was then able to confirm that Sasuke did indeed have such an ability which I personally believe that Indra had. Just like his Rinnegan, I think that while Madara had to take Hashirama's chakra/cells to awaken it Sasuke was born with the innate ability to awaken it once his eyes reach their full potential. That's why Sasuke was more special than Itachi and had the potential to surpass Madara and maybe even Indra, as non of those was able to awaken Rinnegan on their own
