Not to play Devil's Advocate or anything, 'cause I think the show does make it clear that Smallville (the town) is the "meteor capital of the world," but, Lionel was pointing out that the
KEY was unique, not meteor rocks. Here's the quote:
"The composition [of the key] was unique. There's nothing like it on Earth, with one exception: the meteor rocks found here in Smallville."
He doesn't really say specifically that the meteors are ONLY found in Smallville, just that the composition of the key matches that of the meteors found in Smallville. It's a statement of inclusion, not exclusion, so it's merely consistent with what they already know. It doesn't prove something they don't know - like exclude the existence of Kryptonite in other places such as China, South America or even Addis Ababa LOL. So the statement is somewhat of a grey area, if you *really* wanted to debate it.