Not always. "I don't know" is identical to saying "I have a uniform prior". In the particular case of LK-99, my prior is perfectly uniform - I know absolutely nothing about the subject matter and almost everything I read on the topic feels way above my head. As I refuse to be influenced by experts (or "experts") until there is some sort of consensus, I have absolutely no good reason to say anything but "I don't know".
A perfectly uniform prior suggests that you think that it's easy to make a room temperature super conductor and that people have made them before with equal likelihood to it being hard to make a room temperature superconductor and people haven't made them before, doesnt it?
I don't know much about superconductors, but I do know that people have been trying to make them work at room temperature, and generally failing
[1] https://slatestarcodex.com/2013/05/02/if-its-worth-doing-its...