In watching players over the decades, it seems to me one could reasonably suggest that there are lots of different techniques that fall under the banner of "slap". And thus it would seem to follow that the best string is going to be one that fits your particular technique.
When I was playing in the fusion group a decade ago, I used the same bass with the same strings for everything we did. (Well, for a while the guitar player played some songs with a guitar tuned down a half step, so I had a second bass on stage for that; but it too had the same strings.)
There was one song in particular where slap was a focal point - Chameleon; and I thought my strings were fine for my approach to using slap to carry the song.
Same strings I'm using now - Thomastik Infeld Jazz Rounds. And they are very light gauge.
My bass, strings, and rig gave me what I thought was a great tone, with great playability. But tone is subjective. Also, I wasn't trying to sound like anyone else; I wasn't trying to replicate the tone that was on a record.
But yes, I can certainly see where some players would want a heavier, stiffer, string for playing slap.