Sometimes, forcing someone to answer the very question they have posed is a great way of pointing out any flaws.
The taking of offense here is the bigger enemy to progress than anything else in this exchange, and that didn't come from the interviewee.
I'm all for people being cordial and polite, and many programmers could learn a thing or two about human interaction, but the idea that asking a question in response during an evaluation of capability (on both sides) is an insult is a little bit of a stretch.
Others here seem more interested in giving snarky half-truths.
You don't have to be an argumentative dick to express doubt in something. Nor are you necessarily an underconfident yes man if you dont speak up. Circumstances dictates a lot of behavior.