This would essentially require a PoD long before the 8th century BC, which might in turn render the alternate East Asia entirely unrecognizable from that of IOTL. Specifically, the chaos from the Spring and Autumn/Warring States Periods may have caused migrants from China to flee to the archipelago, while later developments led to friction between the Yan and Gojoseon, in which Liaoxi was conquered from the latter, triggering migrations southward into the peninsula, pushing many of the inhabitants eastward into Japan.
In order for the widespread chaos to be butterflied away altogether, China would have had to remain stable for a far longer period of time. This may well have been impossible as remote frontier regions began to assert their independence from the Zhou, making it essentially inevitable that neighboring entities would eventually go to war with each other.
Also, Japanese toponyms are only found in areas south of Pyongyang, with none in Manchuria, which has a high density of Korean ones. Genetic research also suggests that the respective populations in the peninsula and the archipelago may not have changed significantly, suggesting that founder populations gradually assimilated local inhabitants.