I've only found this saying on Chinese websites. I've never heard it before now. In spite of that, it's still understandable: there is a partner for everyone on this planet. John is a boy's name and Joan is a girl's name, and both names sound similar. They almost match.

I think so it means every man found his companion.

In Polish we say "Każda potwora znajdzie swego amatora" (literally: Every monster will find her lover) which I guess is pretty close to the meaning of your sentence in spite of abuse included in Polish ;)