He both invented and discovered it. Yes, the object literal syntax existed, but he also carefully (and IMHO correctly) specified a strict subset as well, for these interoperability reasons. For instance, Javascript is happy with {a: 1}, but that is not legal JSON. It's a very well done standard.
Indeed, and I apologize for my ambiguity, as you are correct. By "strict subset" what I meant was a subset that attempts to reduce options, so that legality and illegality is easier to discern. That is, where Javascript accepts apostrophe and double-quote to delimit strings, JSON only accepts double-quotes, thus, "stricter" than real Javascript.
You are of course correct that JSON turns out not to quite be a strict subset in the set theory sense of "strict subset", though obviously that's a bug in the spec rather than a deliberate design decision.
