I have seen nobody claim this, and this may simply be a matter of collateral damage, as Trump has officially approved of the recent openings related to the forthcoming Winter Olympics in South Korea, especially since South Korean President Moon went out of his way to give credit to Trump publicly for this opening, credit Trump publicly accepted. It is probably more that Trump simply is not thinking, but it is also the case that he had previously dissed moves by Moon to follow a more peaceful approach to North Korea.
For anybody who does not know, South Korea is the leading source of washing machines imported into the US, so South Korea will take the largest hit from the recently announced by Trump tariff hike on washing machines. It may have nothing to do with South Korea at all, with the fact that domestic producer Whirlpool being located in politically sensitive Ohio that played a major role, with Dem Sen. Sherrod Brown praising Trump's move (and the tariff on solar cells probably part of his ongoing effort to help domestic fossil fuels, especially coal).
OTOH, Trump has for some time been railing against the free trade agreement with South Korea, even as South Korea has been under this war threat from North Korea. Trump preferred disgraced former President Park to leftier current President Moon. But aside from his general protectionist MAGA theme, probably the most likely reason Trump has fussed about the Korean agreement rather than some other strictly bilateral ones is that it came about while Obama was president. And we know well that he seems obsessed with undoing every jot and wiggle of anything Obama did. That this would harm a nation his foreign policy actions have been putting under serious threat has not slowed him down much on this, although this washing machine tariff seems to be the first actual move against South Korea economically.
In any case, probably this move was not primarily driven by a desire to punish South Korea for being to wussy soft with North Korea, but it is certainly consistent with such a motive.