it sounds like that first note is a hammer on, but the string is hit with the thumb. same with the rest of the pattern as it sounds. when riding that F#, keep hitting with the thumb, and that "thud" is a muted note. theres a quick pop on the E and hammer on to the F# (an octave higher) before the pattern starts over.
of course my bass doesnt sound like whats recorded, but it grooves when i do it this way. it sounded decent fingerstyle as well, so it maybe more of the timing and rhythm, than the exact slap technique that gives the song that "justice".
hope that helps in any way.
