If you have scathing feedback for someone you are interviewing, you should probably just shut up. Or at least deliver it in a different context where they aren't being held hostage and having their every reaction scrutinized. Sounds miserable, I seriously doubt they were actually "generally positive"
Would HR advise you to give this kind of feedback to someone you're terminating? Normally you say zero for this exact reason.
“Can’t complain!”
I learned it the hard way that even if someone asks for a feedback be very polite and simply convey that the HR has instructed you to route this via proper (HR/recruiter) channels. In fact in some cases, we were specifically asked to exactly say that and often our recruiters set these expectations with the candidates in advance (I've been informed of this in advance before some of my interviews). In one case a candidate kept mailing my team recruiter that the interviewers (a colleague and I) were very happy with the performance and why they were still not getting the offer.
If I want feedback (and this is only when the interview process was good irrespective of whether my performance, as per me, was good enough for that role or not), I ask for a a feedback via the recruiter and in most cases I don't get a response but that's fine. The last thing I want is feedback from someone who, let's just say, is not very ideal.
Another thing is, unless the candidate is someone who is giving an interview knowing they will get 11 offers out of 10 interviews they might be on the backfoot, a bit defensive, even stressed, or scared, and may not downright tell the interviewer that they are being an ass, or insensitive, etc.
It also doesn't cover the case of junior interviewers interviewing more experienced people either by accident or by design.
There are a lot of failure modes that could simply turn into useful conversations with this approach I think.
"This is utter shite"
"Thank you chef!" (Proceeds to burst into tears as they walk away).
I don't think it should be emulated.