Yeah sure, if something is not written down it's not true... typical academic mentality. What if Satoshi just told him? what if it was in an email on an IM that is private and Gavin does not want to publicize it.

I remember my brothers GF. We had a little discussion going on and my argument was based on the law of conservation of energy. She honestly said "well, and how can that be true if you cannot name a book or any academic publication where this has been written down?".
I said:"Look, if you rub your hands together quickly, the energy of you moving your hands will result in them getting warmer". I had a lot of other everyday examples but she always said "If you cannot show me where it's written, you cannot use that as an argument".

Well, clearly she'll never invent anything or be part of any scientific finding or new theories, but thats another story.

The point is, we're neither in a university or an academic environment, nor do we need to know everything with 100% certainty in order to not deny it's existence.

Gavin told us what satoshi said. He chose to do it the say he did. It is up to him to add proof in any way, shape or form to his claims just as it is up to everyone else to believe him or not. This quote can easily be found via google, which makes the source available to everyone.

And last but not least:
Everyone understood it was a satoshi quote and people could find the source by themselves with ease. Therefore, his communication was successful and there is no need to add anything!