Either Jang Bogo would find a way back to Silla before he died, or he would essentially lose contact with his home country. While he depended on maintaining a reasonably sized fleet in order to patrol the waters, this would be extremely difficult to maintain in such a new and (intially) hostile environment. After most of his fleet would be scattered, he would first have to convince a significant amount of natives in order to rebuild his fleet, which would be extremely unlikely. The fact that he would be vastly outnumbered when he would arrive shipwrecked almost certainly guarantees assimilation into the local populace, or death, within a generation or two.
This doesn't work at all within East Asia. The Chinese conducted heqin until the 8th century, in which Chinese rulers married off princesses to nomadic chieftains, but the peace was only valid for the duration of the marriage itself. As a result, it required various marriages in order to maintain even the semblance of cordial relations, and Goryeo would be much more concerned with protecting the northern border and trading with its closest neighbors than sending more further south.
In terms of Korea, Silla and Baekje rulers sporadically married the others' princesses, but the peace only lasted for the duration of the marriage, not to mention that it was frequently broken. Baekje attacked Silla consistently throughout its existence, most notably in 399 when it sieged Silla's capital, while Silla finally counterattacked in 553 by breaking the previous alliance against Goguryeo and seized the Han River Valley. Later, Goryeo commoners married Mongol Khans, and Mongol princesses married Goryeo rulers, due to stringent requirements in order to maintain the "peace," but succession was not affected in any way. The arrangement also did not prevent Gongmin from maintaining an aggressive stance and attacking Liaodong, even though he was essentially Mongolian in terms of ethnicity after numerous marriages.
The marriages would have to be consistent for the "alliance" to be maintained in any way, which would be virtually impossible considering that Goryeo would already have its hands full with numerous other issues. If the state was concerned with either defending itself from substantial incursions, or retaining trading relations, it wouldn't think about sending anyone further south, not to mention that "marriage alliances" were virtually nonexistent in general, with very few specific exceptions.
In addition, Kim Suro probably married an Indian princess, but there is essentially no mention of close ties between the states, most likely due to logistics.
The issue isn't whether the story itself was fictional or not, but whether an alliance could be maintained at all, which probably won't happen regardless of the circumstances.