Myself and several of my family members have had post-interview chats (formal and informal) like you describe on different occasions. In those chats we learned things like:
- The interviewer had a different, not-yet-posted position that they felt would be a much better fit and wanted me to apply for that instead.
- The interviewer thought I was a great fit but while they were in the process of filing the paperwork to make a formal offer, their budget got cut and they no longer have the resources to hire anyone.
- The person who got the job was a truly rare find that was significantly more qualified than me (I would have hired them too), but that I was the runner-up and would have been successful in the position if hired. They said that the next time a similar position was posted, I would likely get it if I applied.
- A recruiter informed me that my candidate profile in their applicant tracking system was suspicious and nonsensical, leading to automatic rejections before a human ever saw it. She looked into it further and noticed that their system couldn't parse my resume correctly due to the fancy formatting, and helped me re-format it and get past the automated screening systems.
- The interviewer (who was a coworker of mine around a year prior) let me know that he intended on hiring me, but the legal department vetoed it due to internal anti-poaching rules. He said I could easily get a similar job at that company on a different team, or even get that same job if I waited several months for the anti-poaching window to expire.
- The interviewer was familiar with my skill set and quality of work and thought I'd be a perfect choice. They let me know that I really needed to get certain industry certifications in those skill sets in order to be competitive even though they weren't technically required. Hiring committees will almost always favor people with those particular certifications, even if the non-certified candidate clearly has more actual skill and experience.
The common thread in these is that the interviewer believed that I could do the job well despite not being selected, and was helping me be more successful (and by extension, helping themselves by making it more likely they could hire me in the future). HR people are fairly well paid, and they won't be wasting their time making these types of calls if they weren't worth it. If they really didn't want you, they'd just send a rejection form letter and not waste another second on you.
Of course, it's possible that you could end up in a call that only exists for CYA legal purposes and is a complete waste of your time. But it could also contain key information that turns your job search completely around. For me, that tip about fixing my resume was the end of a grueling, year-long job hunt with zero success.
If it were me, I'd definitely take the meeting. Worst-case scenario, you play solitaire for half an hour and then move on with life. Best-case scenario, you learn something that completely changes your job search (current and future) for the better. It seems far too risky not to take the meeting.