Low action helps A LOT, but from trying his strings I can also say that they are very bouncy and low tension: a dream to slap. A bit too scooped sounding for other techniques, IMO, but no surprise they sing with slapping.
I think he has a fairly low action. You can hear that when he plays fingerstyle. The string snaps almost all the time, which is kinda cool. But his bass is set up mostly for slap and melodic playing.
Larry Graham already played double thumbing in the 70's by the way, long before Victor and Marcus.