Does that mean they don’t have knowledge about such things?

It's more complicated than "yes" or "no".

Scientists are better justified to claim knowledge over those things due to reasoning; reusing your example, black holes appear as a logical conclusion of the current gravity models based on the general relativity, and that general relativity needs to explain even things that scientists (and other people) experience directly.

However, as I've showed, LLMs are not able to reason properly. They have neither reasoning nor access to the real world. If they had one of them we could argue that they're conscious, but as of now? Nah.

With that said, "can you really claim knowledge over something?" is a real problem in philosophy of science, and one of the reasons why scientists aren't typically eager to vomit certainty on scientific matters, not even within their fields of expertise. For example, note how they're far more likely to say stuff like "X might be related to Y" than stuff like "X is related to Y".