If so then that means the training objective is wrong because admitting you do not know something is much more a hallmark of intelligence than any attempt to 'hallucinate' (I don't like that word, I prefer 'make up') an answer.

There are plenty of exams that give either partial credit for "I don't know" or negative credit for a wrong answer (to discourage guessing).

Training on internet comments is going to make "I don't know" rather unlikely because when someone asks a question in an online forum and I don't know the answer, people will just not respond rather than responding "I don't know"

I've had very good luck with a follow up "Is that answer correct?"