This seems like a very slippery slope. Can you have ATC question any request that doesn't make sense if it's requested from a pilot currently flying over FL100?
And ultimately, what can ATC do?
Even in this situation, ATC cleared the pilot down to FL200...the pilot acknowledged, yet never descended from 250.
We don't know what the "incorrect indication" was the pilot noticed...but it drove him to request a lower altitude...that tells me it was likely a correct-indication of a pressurization problem. Again...just like they tell the pax..."put on your own mask before helping others..."