Heh, I've had this happen twice now with Google.
The first time was for a role I actually applied for. I was honoured to even be considered, but when the interview went similarly to how you describe I was rather gutted.
The second time I received an email and a subsequent phonecall out of the blue. Again I was excited and told that it'd be an 'informal chat regarding opportunities here at Google'. The call started off great - being told that you could work from any office all over the world, employee benefits, exciting-sounding projects, etc. etc.
Then I was asked to rate myself on a "scale of 1-10, where 1-5 you have some knowledge on the subject, 6-7 you're considered an expert, 8-9 you've published work in this area such as a book, and 10 you're the person who wrote the language or you're considered a world authority on the subject." And from then on in it just descended into madness.
How does an average, even an above average person, ever rate themselves - if they're being reasonably modest - above 4? "I see you're keen on Python, how do you rate yourself there? By the way, you know the guy who invented Python? Yeah, he works here, heh heh" and so on. Following on from that little taster (which made me feel approx. 1" tall) we then had a 'quick fire' round of relatively obscure questions. A held my own here but given how my confidence was undermined at this point I don't think it was especially representative of myself technically.
And that was that. The only thing that I can take away from these is that I'm not the 'type' of person Google are looking for. Good luck to anyone who is.