I am trying to remember what nonexistent period this is, given that Japan as a unified country was a threat only twice: Imjin and the annexation. Neither were a east-west divide.
I think "always" here is referring to the modern era, seeing the reference to trying to balance influence in Korea the way the late Joseon did.
But there was plenty of factionalism even in the old days, naturally. Gyeongsangdo and Jeollado have been antagonistic as long as most people remember, especially given how prominent the former is in Korean politics (pretty sure the majority of Korean presidents are from the southeast despite that region being, what, a quarter-ish of the total population?). Honestly, I'm not sure if that's a new thing or the reopening of an old rivalry (heard someone refer to it as a continuation of the old Silla and Baekje dispute) but there's always been a regional divide thanks to the mountainous terrain. Always disagreeing on something, it seems like.
The whole foreign influence thing though, definitely more recent. There wasn't a pro-Japanese faction until the Meiji Restoration and, even then, there was the Russian faction in addition to the Chinese and Japanese factions (a shrimp between whales then, yes. I don't think any late Joseon officials were delusional enough to think they were capable of opposing any one of the empires around them without one of the others supporting it).
And seeing as Korea had been unified and centralized for over a thousand years, longer than most of the Great Powers had even existed, I don't think division was even in anyone's mind until the trusteeship at the end of WWII. Might be wrong but the Korean independence fighters definitely didn't expect that.