I know this feature was working at release time and for some time after, but as of right now this specific bonus isn't working. No mods enabled, starting a fresh colony with a Waster pawn, rolled a new world, embarked on a polluted map tile, walk pawn over some polluted soil and while the other buffs seem to apply correctly, the movement buff does not.
(https://i.imgur.com/jIu0bBy.png)