In the album’s personnel, Flea is listed as playing bass on that track. I don’t hear anything that could be additional bass, so the main bass must be played by him. I agree that it sounds vocal, but I think that the deliberately reduced quality of the introduction pushes it in that direction.
Of course, it is possible that vocals were layered with the bass guitar, but unless we get some sort of confirmation by the band I am going to assume not.