Well, we can't give Mr. X credits for that because we don't even know who he is in fact.
I KNOW!! Believe me, I've searched for him. 'Quai is one of my foremost bass inspirations, and that track is my favorite of theirs. Unfortunately, it looks like we may never have a satisfactory answer after all the speculation, false claiming (Joeri Hommerson, that fraud) and Jay Kay's drugged-out fuzzy memory.
A few of my own personal theories as to who he may be:
1). Jaco himself, back from the dead (or from that hidden island where Elvis, Tupac et. al. ended up), ripping it up for one last, glory-filled moment in Europop!
2). John Entwistle, taking a walk on the wild side of bouncy fretless funk.
3). Barack Obama. The man can do anything, right?

(Ok, ok, not trying to get political or anything, I don't actually give two shakes about Obama or any other politician)
4) Sid Vicious.
5). Jay Kay, playing with only his feet.
6). Stu Zender himself, in a brilliant but uncharacteristic moment, playing better than ever before or since!
And thanks, CapnSev, for that killer video. Can't really get enough of Stu either!