You could say the same for any product that they can help you find for a cheaper price..
Monzo’s not going to have the capacity to plug into the APIs for every product (if they even exist) to compare the prices for you.
In other words, I don’t think they’ll have access to the product level data. The closest I can see them getting is: we see you spend £x per month at a merchant that sells medicine, this service might be able to save you £x on your prescriptions. But that’s a little too close to untargeted advertising for me. Switching gym memberships would be more likely..