The assumption being that science (and therefore man) is capable of understanding everything.
I disagree, or at least I disagree with the idea that we should assume that is true. Man is product of evolution, the engine of evolution is arbitrary genetic change. I find it unlikely that something that has evolved as a result of arbitrary genetic change should attain such a degree of sophistication that they could understand absolutely everything.
It strikes me as superficially arrogant. No wonder Monty supports this view.