WRT Korea this idea of the "hermit kingdom" is because of a Eurocentric worldview.

Korea into the 19th century had a flourishing relationship, both diplomatic and commercial, with both Beijing and Edo. After the Jurchens and the Chinese merged into a single actor in the mid-17th century as the former conquered the latter, these were the only two neighbors of Korea that actually mattered. China, Japan, and the Jurchens were also the only neighbors that had mattered much for Korea ever since around 1387. So "the hermit kingdom" wasn't an isolationist hermit kingdom at all, it was a kingdom that followed established diplomatic and commercial protocols with established neighbors.

It is true that early 15th century Korea was more "international" than early 19th century Korea in many aspects but not all. For example, early Choson Korea had some form of contact with Thailand and into the 15th century there was a small Muslim community that eventually assimilated into the wider Korean population. But Korea's ties with Thailand or even the Ryukyus were totally inconsequential. What really mattered was that an amicable relationship was met with their immediate neighbors.

So really "the hermit kingdom" just did what it had been doing for centuries - maintaining good relationships with important neighbors they already knew.