I'd say that 양말 한 짝 is one sock (of the pair) and 양말 한 켤레 is a pair of socks
You've gotten to the heart of what makes Korean difficult. The meaning of many words is really a kind of feeling that corresponds to the situation in which the word is used. So for example a 산책 짝 would be a walking partner. Meaning, one person to round out the pair. In the sense of socks, it often means the pair itself. But if you've got only one sock (and the other is missing) you could actually say "이 양말 짝이 없네" - this sock is missing it's partner - and it would totally make sense. Fun language.
aaaaaacctuuuualllyyy.....it's better to think of it as meaning "match" that way the sentence says it found "one match" which means it still needs the other.