I like it - although pasting garbage in the field gets the same response as (presumably) a real private key. Doesn't specify what kind of private key either.
Thanks for pointing it out, all the fun and irony aside I bet someone might not get it, so I think we should be responsible and make it explicit that you should never, EVER post your private key anywhere. If you click "Check" it says:
"Why on earth would you post it here? Now it's surely not."
SERIOUSLY. THIS IS A JOKE.
DONT POST YOUR PRIVATE KEY ANYWHERE.