Based on the response he got, it was the right question, actually. People aren't Internet-style insecure in real-life, especially those who have high social cred (like doctors). Even accounting for the humorous exaggeration, the kind of question asked from a professional doctor is less likely to cause them to be offended and more likely for them to just tell you why not. They're not going to be "How dare you question my decades of experience?!". They'll answer like they did in the OP.
In The Mom Test, he suggests getting right to the core of a customer's pain points. This is just corollary to that.
Yeah, I do clinical placements in hospitals as part of volunteer work, and doctors run the range from open and receptive to questions to incredibly arrogant and dismissive.
The fact that they agreed to talk to the OP probably means they aren't going to be immediately dismissive, but I'd hesitate to assign any supranormal human traits to doctors.
In The Mom Test, he suggests getting right to the core of a customer's pain points. This is just corollary to that.