a further explanation provided by another person if you want it:

There are several basic errors contained within the tweet thread (and in the screenshots), but the biggest tell is that the source claims the mobilization of only six of fifteen divisions by North Korea in their initial offensive is proof that North Korea could not have started the war. This is, of course, nonsense and the much more likely explanation is that mobilization is difficult particularly with underdeveloped infrastructure. Hardly any country ever met the full mobilization tables on time. There's also some intentional misrepresentation of quotes: Gunther quotes Syngman Rhee stating "We started the fight in the first place in the hope that Communism would be destroyed" as proof that South Korea started the fight. This is again absurd, this quote is pretty obviously saber-rattling and rhetorical flourish and not some indication that Rhee literally started the fight. Similar quotes can be found by leaders of nations at war in WWI and WWII that very obviously did not start the war in question. Finally, they cite UN assessments that North Korea couldn't have launched an offensive in 1950 as proof. This again is absurd. The UN and its predecessor The League of Nations had repeatedly failed intelligence estimates of force and still regularly fail to anticipate conflicts!

The overall sense is that this is someone who started with a thesis (and, unsurprisingly, the thesis seems to be USA bad, enemies of USA always good and blameless) and subsequently found sources that could be interpreted to agree with the thesis, rather than looking comprehensively at the evidence and making a thesis from that.