If I were still in my 20s I might have fought Wikilawyers on this. Now I'm old and I have enough shit to take care of. :/
In my experience, the vast majority of my edits were accepted when properly sourced, especially on esoteric topics like this. Yes, there are exceptions and bad apples in the community. Those get a all the attention and media coverage but I've only encountered them on heavily edited and visited pages, not the more rare academic or historic pages. While it's anecdotal, I just checked and I've clocked nearly 800 edits in the past decade.
Thus I'd imagine that of the small number of people intimately familiar with Korean history who even noticed, very few could say with sufficient certainty that the flag was never used.
(For that matter, https://en.wikipedia.org/wiki/List_of_Korean_flags has a third variant...)
I would be more charitable if it wasn’t for the fact that 99% of all claims of misdeeds on Wikipedia never give any links, or even sufficient details to find the offending edits.
I generally support the need for evidence, but I have only so much time. And I saw many, many instances of this behavior.