Those maps have come out from time to time but for the most part, we don't really care about them.
In fact it is in China that whenever such maps or crazy claims are made they are blown out of proportion. Lately, fabricated and fake "news reports" apparently written by Korean journalists making outrageous claims about the grandeous nature of Korea have been found to be made in China by internet flamers like yourself. I can't find the link right now because it's been a few months since it happened but I'm sure it's only a matter of time before it happens again so all we have to do is wait.
Truth is, I don't understand why some people make those claims. In history, Koguryo (one of the old Korean kingdoms) did have a lot of land in present day Manchuria that were its own territory and was possibily or probably born in that region but that was hundreds of years ago and I don't see how that is relevent anymore. Even today millions of ethnic Koreans live in those areas.
The answer CLOSER to the truth is that Korea lived as a vassal state to various Chinese dynasties and therefore claims that China used to own all of Korea is far more valid.
But what counts is that it isn't the reality today, just as Manchuria is no longer a part of any Korean dynasty or republic and hasn't been for hundreds of years.