Their theory is that there is a scientific law of nature called the Wave Function of the Universe that implies that it is highly probable that a universe with our characteristics will come into existence without a cause.
First, the scientific law says that the universe would come into existence because of its natural, mathematical properties, not because of any supernatural forces.
My argument will proceed by showing how, given this common view, it follows that it is better never to come into existence.
This judgment is too strong because if the absence of pleasure in scenario B is "bad" rather than "not bad" then we should have to regret that X did not come into existence.
But so long as there are some negative aspects, the life is not preferable to never having come into existence.
Following from this, there is a difference between saying that it is better not to come into existence and saying how great a harm it is to come into existence.
The fact that one enjoys one's life does not make one's existence better than non-existence, because if one had not come into existence there would have been nobody to have missed the joy of leading that life and so the absence of joy would not be bad.
Now it cannot be the case that (all things considered) it is both better to have come into existence and better never to have come into existence.