^This.

The diamond sutra was written around 868....long long after the Buddha had passed away.

[URL unfurl="true"]http://en.wikipedia.org/wiki/Diamond_Sutra[/URL]

This particular sutra is meant more as a koan to get the person to break down the ego.  To an enlightened person there is no self...which would make the diamond sutra correct...  To be sure this phrase wasn't intended for everyone to take at face value.  An unenlightened person claiming there is no self is delusional.   

Furthermore enlightenment is possible and many people have achieved it, and many of them have said so...including the buddha and quite a few of his close disciples. 

I recommend the book "In the Buddha's words:  An anthology of discourses form the Pali Canon" for anyone that is seriously interested in learning more about Buddhism.  Buddhism is an incredible deep subject and there is pretty much no way someone will understand the grander meaning without actually walking the path.

Peace.