yep. I doubt they cared about the drug supply thing considering the benefits they were getting out of the arrangement. Well, the benefits they would have gotten had Picard not sabotaged the operation and turned the Son'a into outright allies of the Dominion rather than merely suppliers.
Becuase a finite source of magic youth particles really helps with getting shot by a weapon set to kill or being exploded
Meanwhile there's really nothing to stop the So'na from sharing their particles with the Dominion meaning, Yay advantage nullification.