There are some technical limitations like this to the NPC action system, so not everything an NPC says can actually be handled by the game's logic.
It's a matter of every "vague" NPC action like this that can't easily be narrowed down to with conventional natural language processing or game conditions costing about ~0.25s of GPU time (almost) every dialog turn, since the AI has to be queried if "Celandine offered Player free access to the bathhouse" is true.
There are other things the NPCs can do that are much more niche, like rotating furniture or chopping trees, yet those things are included in the game since they can be very precisely narrowed down to without querying the AI.
For reference, if you haven't already discovered, getting Oriana or Celandine to ~70% affection or so makes all your future visits to the bathhouse free forever. Also something you can try if one of them does offer you a free visit in, just ask them to give you the 5 gold to pay the entry fee, they probably will.