Yes, that it is true. Altho Stanley did "pave the way" for a lot of thumbing in jazz, most of what sounds like slaps and pops on his recording are in reality him playing fingerstyle very aggressively. It is a technique both Stan and MArcus Miller use a lot, and call "hard plucking".
Stan is one of the very few bassists who can pull off 32nds and even 64ths fingerstyle (too make matters worse he can perform 32nds on the URB!). His touch, agility, lightning fast articulation coupled with the Alembics he uses and his higher tunings all amount to the "faux slap" tones usually associated with him.
I remember a few years back seeing the Grammy's and Stanley perfromed a duet with the late (and great) Chet Atkins,CGP...and the two of them traded off some incredible licks and phrases, with Stanley matching Chet on every one with that "signaure" hard plucking sound.
MAx