Earlier quoted context omitted.
You probably would not get hired.
You probably would not get hired. Probably not. But what does that say about this particular technique? There's the ability to explain something, and there's the ability to perform under sudden pressure. I would give candidates some period of time (say, 10 minutes, if they needed that long) to select a topic first.
Given the option to talk about anything, I'd be concerned if the candidate couldn't pick something in 10 seconds. Summary of last book read, semantic difference between method calls in C++ vs Objective-C, sewing, recreational ballistics, sociopolitical solutions to poverty, ... something? I could spend the allotted 5 minutes just reviewing topics I could go on for 5 minutes about.