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They aren't completely separate skills. If you have two right-handed hitters and both of them are "true" talent .280 batters against right-handers, then you should expect them to hit lefties about equally well regardless of how they had hit lefties in the past.
Or let me put it this way. If you have two righties who both have an .800 OPS over the last 3 years, with one having a very large platoon split, the largest in the game, and the other having no split at all (he had an .800 OPS against both lefties and righties), what should you expect going forward? You should expect them to have essentially the same split going forward. That means it's very possible that the guy who had a huge split could suddenly have almost no split and the guy with no split have a giant split.
Take a look at Edgar Martinez. In most years, he would hit lefties better, sometimes dramatically so, than righties. But in a few years, like in 1998 and 2001, he actually had a reverse platoon split, doing a lot worse against southpaws than righties. That's why we should just focus on a players totals instead of slicing up his numbers into smaller samples that have more noise.