I suppose it needs the context - out of context, I can't see how it demonstrates anything except that the man could take seven away from ten and get the right answer.

Arguments aren't just about logic and reason, they are also about winning opponents or neutrals over, and there are a whole set of elements that enter into that. I don't think the word fallacy applies to all of them.

For example, if someone picks their nose while making their case, this doesn't in any way weaken their argument in rational terms, but it is likely to reduce their ability to convince others. (Of course that doesn't apply online, unless you're using a webcam.) But I don't think refraining from picking your nose can properly be called a fallacy.