It is still a hallucination even if the words it hallucinates happen to line up with a factual sentence, in the same way that a broken clock happens to correctly display the time twice a day. The function of the clock does not suddenly begin working correctly for one minute and then stop working correctly the next. The function of a broken clock is always flawed. Those broken outputs, by pure coincidence, just happen to be correct sometimes.
LLMs are broken in the same way. They are just predictive text generators, with no real knowledge of concepts or reasoning. As it happens most of the text it has been trained on is factual, so when it regurgitates that text it is only by happenstance, not function, that it produces facts. When it hallucinates a completely new sentence by mashing its learned texts together, it's pure chance whether the resulting sentence is truthful or not. Every generation is a hallucination. Some hallucinations happen to be sentences that reflect the truth. The LLM has no ability to tell the difference.
LLMs are broken in the same way. They are just predictive text generators, with no real knowledge of concepts or reasoning. As it happens most of the text it has been trained on is factual, so when it regurgitates that text it is only by happenstance, not function, that it produces facts. When it hallucinates a completely new sentence by mashing its learned texts together, it's pure chance whether the resulting sentence is truthful or not. Every generation is a hallucination. Some hallucinations happen to be sentences that reflect the truth. The LLM has no ability to tell the difference.