It's common enough for people not to know the difference between a tag and, well, any part of the syntax of HTML that is not a tag. That doesn't mean we should encourage this confusion. I see this frequently in technical channels where people have HTML questions but lack the vocabulary to express them or comprehend the answer. One of the very first things an HTML newbie must learn is the correct names for elements, tags and attributes, which usually means unlearning the incorrect use of 'tag'.

Is this a nitpick? Perhaps if your subject had been something other than markup, or valid markup in particular, you'd be right. I think it's reasonable, though, when writing on a technical subject, to insist on correct usage of that subject's basic technical vocabulary.