I found the linked answer by @TFD in the "Low Quality Posts" review queue. The text up top says, "This answer was flagged as low-quality because of its length and content."
I don't see anything wrong with either the length or the content of the post. On the contrary it is far above average quality (in my subjective appraisal).
Maybe the "Low Quality Post" selection code throws in the odd random post to keep us on our toes. More likely it's a bug though.