Disclaimer: This is opinion.
The answer as it stands is not entirely incorrect. It is in fact somewhat right, as J.R. noted in the comments. I see where you're coming from on that point, but I don't really think you can argue against the answer on that basis.
However, it is an extremely poor answer: It explains nothing, it repeats itself for no reason, it provides no sources, and it is needlessly dogmatic. (When writing a letter, "correctly" using off vs off of is nowhere near as important as basics such as spelling the recipient's name correctly.) On that basis it would seem to be a perfect candidate for the VLQ queue.
Note that there is no flag reason stating that "the answer is wrong". This is by design. Wrong answers are supposed to be downvoted. Moderators (or those with access to moderator tools) are not supposed to be judging the correctness of answers in their role as moderators. George Stocker posted an excellent answer about correct use of VLQ on MSO. Shog's comment on the answer is informative as well. Finally, this duplicate question on MSO has some useful answers, including one from our own mistu4u.