Is calling an individual user or their post "pedantic" a CoC violation?
Yes. That said, the specific comment you're talking about is not the user that's making the comment calling you or your post pedantic. It's attempting to read the minds of those who downvoted instead, ascribing these thoughts to them. Which may be arguably quite useless, but I would not call it a CoC violation.
In addition to that, like Shadow already said, sometimes there are no better words to explain a downvote either, and not explaining a downvote is seen as unfriendly too. For the sake of brevity and staying within character limits, sometimes 'pedantic' just works better than 'excessively focused on minor details to the point of it becoming annoying'. And there's a previous comment that does do the more verbose thing:
Not my downvote, but I can see why one would downvote this. The only difference is the switching of places between Next and Cancel. It's not like that makes this entire screenshot useless, but at the same time there really isn't any need to update them either. If one was busy updating all screenshots, then yeah go ahead. But to go digging for a screenshot that's ˜98% the same.
After that, yeah, summarizing that in a single word (pedantic) in the next comment is probably fine. I would definitely take that context into account when moderating comments, at least.
Then again, nobody here can read minds (I hope), so commenting about why other people may have downvoted isn't always useful if you can give no actionable advice on improving the post. Especially if you've encountered particular users before and you know that they are likely to create a fuss when you leave such a comment, sometimes it's better to just downvote too, or stay silent, and move on...