Sometimes sending the interviewee to lunch with potential co-workers is an optional part of the interview. I've been on many such lunches (mostly as a potential co-worker, only once as an interviewee)
Obviously, it's up to management if they want this to be a part of the interview.
definitely is but I've found that if the manager wants the hire (esp post the company deciding to move forward with an offer), then you have leverage and this is a very sane ask.
Obviously, it's up to management if they want this to be a part of the interview.