Hey Brad-
Back in the day(about '75), I used Labella tapewound strings...FME, they didn't work at all for slap bass.

And this was right when the Bro
Js came out with their stuff, too.
My world changed when I finally went the Roundwound route.
Nowadaze, I'm pretty convinced I can slap/pop any bass with any kinda string(maybe not "well"; that's another story)...I'm pretty sure I had a Technical issue back then, though string technology is now light years ahead of what was available to me back in '75.
Just one man's observation.
Anyway-
In the initial post, it sounds like "P-Nut" is slapping his "G" string. That's OK; my Pre-CBS Fender P-bass didn't like having its "G"-string popped, either...so I used the Thumb exclusively on that bass for this style.
IIRC(& I do recall), I played through either a Bassman or Music Man 65 with the Bass and Treble UP-UP-UP.
I also made sure to strike the "G" string with part of the thumb actually hitting part of the neck...that little piece of wood just past the last fret.
When you're playing a 'log' like I had, you gotta make do.