[kemayo]

I suspect there's a much smaller number who affirmatively know that there's not a specific flag for a kingdom that ceased to exist 1,335 years ago. Particularly since the article presents both a pure red flag and the red/yellow flag and claims they were used at different times.

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...)