This question asks if a statement from a textbook (which looks wrong to the OP) contains a typo or mistake.
It is a mistake, indeed.
I answered “Yes, this is an error. Well spotted.”
… and my answer was deleted. I guess because it was deemed “trivial” (I received no notification, and I cannot write comments in deleted questions).
So the original question and the OP are still waiting for answers.
I'd like to know which would be the right answer to such a question? What more is to be said? Should one add useless characters so that the moderators don't deem the answer trivial?
BTW: it looks strange to me that I get notifications when my answer is upvoted/downvoted, but not when it's deleted (I guess that's because it does not change my rep, but still…)
Edited: What I got by asking in Meta was to get the question itself closed as off-topic. I conclude that, if a math student finds some confusing paragraph in her textbook and wants to ask if it's some misunderstanding on her part or some typo in the book, she should not ask in MSE, because such question will be deemed “off-topic” here. Great!
Edit 2: My original wording was inaccurate. I didn't mean “errata” (as a published list of known errors in an book), but just an error or typo. Fixed.