Not saying I agree that being closed source is in the public good, although one could certainly argue that accelerating the efforts of bad actors to catch up would not be a positive.
Not really. It slows down like security over obscurity. It needs to be open that we know the real risks and we have the best information to combat it. Otherwise, someone who does the same in closed matter, has better chances to get advantage when misusing it.
The whole “security through obscurity doesn’t work” is absolute nonsense. It absolutely works and there are countless real world examples. What doesn’t work is relying on that as your ONLY security.
And the countries that want nukes have some anyway, even if they are not as good.