If I have an interview scheduled to go through lunch hours and the company does not offer me lunch, that makes a very bad impression on me. To me it's part professional courtesy as well as an indication of how well things are planned. Also, if you are able to lunch with the team, it lets you get to know each other in a less stressful situation.
That seemed very weird to me. When I interviewed, it was customary to have a Googler who was not an interviewer take you to lunch at one of the cafes. If you were a referral, it's usually your referrer; if you know someone at Google, they try to match you up with that person. It's your chance to ask questions of an employee in an informal setting, where they won't be making a report back to the hiring committee. And I know they still do it, because I was the lunch interview for a friend of mine that just applied a month or so ago.
The only thing I can think of is maybe they couldn't find interviewers for the normal time slots, and so they had to schedule one at lunch as well. Qualified interviewers are often a scarce resource at Google, mostly because of people like me who never sign up for it, and they try to avoid assigning more than two interviews/week/person to avoid burnout and let them still get actual work done. It may've been a choice between "schedule one during lunch" or "wait another week until a different interviewer is free".
