I have never met any korean nationalists who would go that far with such claims, ofc they would dispute the origin of some cultural aspects, and more than that they would make rediculous territorial claim like northeastern China should be rightly korean (in extrem case also including the coastal region of china), also japan should be destroyed and colonized by koreans. However all of them would uniformly argue that such "create-and-invent-everything" claims are made up by the Japanese (and taiwanese) without any base, the purpose of such fast spreading rumors is to ruin the korean image and sour the relationship with mainland chinese. Despite everything personally I would still trust koreans more than japanese.
I wouldnt go as far as "Nazi", just combine youth and boredom with the wish of finding pride in their own identity, you would find similiar pattern in all the eastasian groups moving rightwing(or leftwing) direction, be it japanese, korean or mongolian and even viets and other southeast asians, they would find delusion faster than any real sign of "cultural glory" from the past. There was a time when I joined such online groups as "submarine" (observation under silence). The occational discrimination against minorities are despicable, but in many cases they just got mixed up;
First the committed genocide against Han citizens by Manchurian (including Han traitors under their command) during the war between qing and ming are considered as crime of external invasion because back then Manchurian were not yet integrated into China properly.
Second the many crimes committed by Han people themselves must not be set equal with those of external invasion, and must not be used as justification for the crimes commited of invaders. In this case I do agree that self inflicted pain should not be be considered the same as pain inflicted by others, for prosecution in most law system created by human mind, without external inference those who is not insane are responsible for their own well being, and free to make decision in case of selfmade mistake against oneself. But deliberate damage made between differet parties must be subjected to lawful prosecution. It is rediculous to see japansese (and some westerners" to justify the crimes of invasion against china based on the "crimes" of communists (like chinese people who died of hunger under Maos leadership). Also in any case civil war conflicts and failure of state policy could never be the same as evil invasions between nations and world wars.
But ofc most of their views are absurd and could even be harmful for national integrity, and like any other extremist idealist groups different expressions would not be recognized, if you dont "dance in the line" there would be instant ban, thats why submarine is the best to enjoy comedy from time to time. Although that is the same case for many nationalist or "patriotic" forums, whatever origin wont matter.