Thanks for asking.
I was the one who locked out comments on that question. You can see this by clicking on the little clock icon beneath where scores are shown, here just to the left of your opening “There's this old question” paragraph:
That gives you the public timeline for that post, which is currently:
The ELU mod team delete what are frankly incredible numbers of comments, mostly in response to flags of varying varieties. Few weeks go by when we don’t delete hundreds of comments, and we’ve deleted between 50 and 100 thousand comments over the history of the site. That’s a lot of work.
Yesterday I responded to automatic comment flags on posts with more than twenty comments and ended up deleting dozens and dozens. Sometimes I moved deleted comments to chat, sometimes I just deleted a few here and there that were no longer relevant or which were answers hidden in comments, and sometimes I just en masse deleted them all. On some posts I thought likely to generate further off-topic comments, I set comment-only locks, usually for a week or for ever, depending.
Yours ended up being one of those posts. Although there weren’t a lot already deleted, it seemed to me like one destined to accumulate lots of highly localized comments that should have gone in the answer box instead. That’s the reason for the comment lock in this instance.