It isn't a hallucination because that isn't how the term is defined. The term "hallucination" refers, very specifically, to "plausible but false statements generated by language models."

At the end of the day, the goal is to train models that are able to differentiate between true and false statements, at least to a much better degree than they can now, and the linked article seems to have some very interesting suggestions about how to get them to do that.