Heh. It's only unfalsifiable if you ignore a person's previous experience and accomplishments, refusing to acknowledge anything other than your own interview as an accurate gauge of the person's skills.
Also, I'm literally telling you that it happens to me. (And I've heard many people say it happens to them.) So either I'm a liar, or you have to admit that it happens. Go ahead and call me a liar if that's what you want to believe, but I'm not even trying to get a job anymore. I'm happily self-employed now, and my customers also seem very happy with my coding skills.
The term "control your emotions" is somewhat vague. You can hide your emotions, but that doesn't mean you're not experiencing the emotions. An extreme example of that is if you "put on a brave face" after a personal tragedy. You may not cry and break down, but that doesn't mean the sadness doesn't affect you severely. The reason I mentioned therapy is that you'd probably need therapy in order to somehow train yourself to not even feel the emotions in the first place in situations where they arise automatically for you, i.e., to not have certain phobias at all.
> the sort of issues I'd associate with nervousness
Have you considered that you might not have a full understanding and appreciation for human psychology and the wide variance among humans?