You are right, but you have to look at the whole situation. If someone is asking what his/her chances of teaching English abroad are, then he should proof read his/her writing.
Same goes for explaining grammar. If you're going to give advice, then you should either (a) make sure that you are correct, or (b) say that you are not sure if your answer is correct. There is no shame in saying that you are not sure/you don't know.
There is also a person on this forum who uses really weird English (bad grammar mostly, awkward sentence construction, strange vocab sometimes), but they will fight you to the end that they are right even though a few people repeatedly prove them wrong. That person likes to give advice, but all they are doing is confusing others.
(and yes, that was a singular "they")
So, context is everything. I agree that it's stupid to point out a silly mistake like a typo, especially if it has nothing to do with the question/answer, but at the same time you have to look at the whole picture and try to understand how (and if at all) that mistake/typo fits.