I suspect that a combination of the Luhn checksum and contextual information would give you a fairly high confidence level of it being a valid credit card numbe

I suspect that a combination of the Luhn checksum and contextual information would give you a fairly high confidence level of it being a valid credit card number, and not some other innocent string of digits. They could clearly mark the stricken text as having been done to potentially protect privacy and provide a contact mechanism to un-obscure false positives. I think the harm of a few false positives -- remember, we're talking about obscuring a 16-digit string that has a particular valid checksum, not a very common occurrence -- is greatly outweighed by the harm of having someone's financial identity stolen from a Google search.