Honestly, I really can't compute how responding to a "I have been diagnosed with X and treatment is going well" comment with a "well doctors misdiagnose all the time" comment could possibly be construed as helpful. What am I missing??
That's the thing, though, we don't know that it was "I have been diagnosed with X and treatment is going well". From the info in OP, it could just as easily have been "Scared and need hugs.. just got diagnosed with a brain cloud!" (points for the reference?)
But is it really supportive to respond, "Oh, doctors don't know anything - you don't likely have a brain cloud, you just have to eat better/exercise more/stop letting a pretend illness excuse your lack of care of yourself"? That's very different from "Maybe it's not as bad as it sounds". Telling someone with a scary diagnosis that they most likely don't have that illness is incredibly dismissive, rather than supportive, to my mind.
If someone has, say, diabetes, or cancer, the best way of dealing with it is not
through denying that the disease exists. A supportive friend asks "what can I do to help?" not "No, you're wrong, you don't have that".