This is incorrect. At least for tech companies, when you are explicitly told that it's not an interview, you can take it at face value (but as always, exercise judgement, and don't be a dick).
Usually they will tell you whether lunch is also an interview or if it's just lunch. From my own experiences, Microsoft typically does a more touchy-feely interview (still a real interview) over lunch, but Google typically doesn't. With the tech giants, you can take this at face value.
That doesn't mean your lunch buddy will be a good conversationalist though. Mine mostly were not (including at AirBnB, but I only point that out for the irony against their stated cultural values). I would sometimes just ask for the last 20 minutes to meditate.
Also that's not an insult or a judgement. Being a good conversationalist in professional settings is hard, and interviews doubly so because interviewees are nervous for good reasons. So I don't judge engineers or even PM's by how good at bantering they are during interviews, because it's not their job to be good at it.