But from a scientist's viewpoint, it would always be regarded as "knowable", as assuming otherwise leads to the end of science altogether
I agree with you. I think it is not possible to state scientifically that something is "unknowable". Indeed I am still not sure if that is even possible in the philosophical realm, because it may not accomplish logical rules.
So, probably, Bohr knowing of that, decided to take a scientific-compatible way and utter that it just doesn't exist. Reality doesn't exist while not observed.
This statement led to new problems: how can the Universe evolve if it doesn't exist meanwhile nobody is observing? How could the Bohr's CI of QM explain the Universe evolution near after the Big Bang? Nobody was there observing it!
I don't know if Bohr had ever the opportunity to answer this kind of questions. I don't know if anybody (contemporary to him) had the chance to ask them to him. I don't know if Bohr eventually reply to Einstein question about the existence of the Moon. I really appreciate any hint about all these subjects.