When I interviewed at a Google outpost, a good-cop employee they mistakenly thought had a connection to me took me to lunch (message: forget about the bad-cop interviewer you were just with, you're among friends, loosen your tongue so our spy can report back) in their cafeteria (message: look at the free food perks you'd enjoy), and initiated a conversation with an visiting economist there who then spoke of something oddly relevant to my research interest at the time (message: look at the interesting people and collaborations you will bump into every day).
Your interview lunch experience sounds like message: this is what it's going to be like, and we don't care if you join us.
I've been on the other side of that (not google), taking people to lunch who are interviewing. Usually you're picked because they're from the same country as you or something like that. At least where I work it's not like those messages. Instead it's lunch because food is good to have, and if you were lunching with them you weren't involved in the interview process at all, and it gave them a person to ask questions of and get an unbiased-as-reasonable response. If there were real red flags I'd probably raise it, but otherwise I had no contact with anyone in the hiring process regarding that person at all.
I don't know what percentage of companies use the lunch in an on-site interview for evaluation, but definitely some do. Either with express feedback into the process, or because it's the hiring manager taking the person to lunch, and this is shaping their opinion.
I suppose some could also use it as a refresher break, from their battery of antagonistic one-way LeetCode hazings, or a chance to give the person a more comfortable feeling about the company (not otherwise permitted by their hazing process).
Your interview lunch experience sounds like message: this is what it's going to be like, and we don't care if you join us.