I'm not sure that's true. It definitely implies that bacta can't be absorbed all the way through the skin into the organs. I mean, did the people who put Brendol in the tank know he was poisoned? If they didn't, then they wouldn't have taken any extra measures.

(should also be noted that Leia went into a bacta tank for a concussion in Bloodline. Not sure where "can fix minor brain things" puts us.)