Well, technically it's not really bold text, it's "strong" text. Markdown actually specifies that the
<strong> element be used to render the text, and a quick look around the web suggests that that means "strong importance" - in other words, text that is more important than its surrounding text. Important in the sense that, if you only see a few words from the whole block of text, they should be these words.
Basically, double-star markup is appropriate for TL;DRs.
So any time the
**...** construction is being used to highlight a short phrase that sums up the post in this way, I'd argue that it's semantically correct usage. An edit that introduces such a change is correct, and one that removes it is wrong. I think the bold text in the answer you're asking about is an example, and it shouldn't have been edited out.
I also think it's such a minor change that it hardly seems worth an edit. If someone puts their entire post in bold (oops, I mean strong importance), or uses it for one sentence out of a two-sentence post, or something like that, then sure, by all means edit that out. Alternatively, if the post is extremely long and there is a sentence that provides a good short summary somewhere in it, then I could see it being correct to edit
**...** around that sentence. But neither of those things appear to be the case here.
Either way, I can't see how the edit that removed the bold formatting was justified. Whether your edit to introduce the bold was justified in the first place, I could see that going either way, but that's not really the point of this question.