Earlier quoted context omitted.
Perhaps - then tell him . The reason does not matter (especially since we are unlikely to hear it from anyone), what matters is that they refuse to say what it is. "Effort" is not an excuse either since they even replied to him - but with something unprofessional while still leaving out the reason for no good reason that I can think of. You can even mask it behind a more fuzzy reason if you think the specific reason…
Disagree. A while ago I experimented with giving job applicants honest feedback on why they were unsuccessful - not just the old platitudes like"there were stronger candidates". Big mistake. When you reject people, they really really want to dig in, know why and then refute or try to overturn that.
That is 101% true. However, when you give feedback you can also politely say that it is not open for a debate.
"We found you lacking in X, Y and Z. We are not interested in advancing our communication. All the best and goodbye."
So IMO just give feedback once and never reply again. If somebody is persistent, block them. Then you would in your full right.
Being fully silent without a single message however is not okay.