I really disagree with that. You might not have time to have code written and completed in that interview. You might have someone who just knows that question but has limited knowledge, you might have someone who can code but has an uncommon failure on your problem. I have interviewed tons of people who can talk about technology but can't do it themselves. If you are hiring people for a coding job, they have to code. People who say they are architects or first level manager will have a really hard time doing a good job if they don't understand the technology themselves (I guess its not impossible, but its harder).
so what do you do to filter people out with such certainty? I worked at a startup, and the ceo was dead certain he could figure it out in a short conversation with someone, if they'd be good or not. he was certain we should hire 2 people, i was certain we should not, i was right both times, he was wrong. But I've been wrong before. Being certain you can do it in an hour probably means you are overconfident.
It's less about whether you want to hire that person. It's which of 100 people you want to hire. You need to have SOME filter prior to wasting 99 hours.