The point of "that is an interesting assumption" is that it is NOT intended to be snarky. It is a neutral statement, meant to disengage. If that's "hyperdefensive", I dont' know what you'd call someone saying straight out, "I'm not a drug dealer, and I find what you said offensive." A declaration of war, perhaps?
If you know enough from this site to consider it snarky, you should know enough to not to be commenting on other people's medical conditions, nor accusing people you don't know, even in jest, of being felons. If I have to carry medication around with me, I don't want to have to be defending myself against accusations of being a drug dealer (and however hilarious you find that, there actually are drug dealers out there, and people can easily find themselves accused for real of being one if they don't take steps to counteract gossip).
I find it very strange that the person who makes the offensive remark in the first place is entitled to storm off raging because they were told, simply, "that's an interesting assumption". That, to me, is "the crazy" that I would be happy not to engage.
"It was just a joke," is the feeblest excuse that can be given for being obnoxious to other people. If you then get your nose out of joint because someone tries a disengage statement on you, you're the one who is hyperdefensive. You should be apologizing for your actions, not raging because, oh my goodness, someone didn't appreciate your humour.