The difference is because the missing word in rhyming slang is just part of the phrase, on itself it wouldn't carry the meaning. "Boracic lint" means skint, and hence so does "boracic". But "lint" on its own does not.

In contrast "foggiest" implies "foggiest notion", but then that is also the essential meaning of "notion" on its own.
..........."

There's no particular reason to assume the Ancient Greeks did not go in for rhyming slang. It would be fun to find out that they did. But I suppose that might be hoping for too much.