You only seem to quote the part you like. If you put your quote in context, he is saying that one cannot connect the
material effect of the beginning of this universe with a
material cause on the "before" side of the singularity.
This is further made plain by the comment below:
"This is that the classical theory, does not enable one to calculate what would come out of a singularity, because all the Laws of Physics would break down there..." [Kind of like equaling both sides of an equation.] "...This would mean that science could not predict how the universe would have begun. Instead, one would have to appeal to an agency outside the universe."
He doesn't like the idea of an immaterial cause, so he interjects imaginary time
in order to preserve causality...but it has to be
material cause. Read this:
"So real time would still have a beginning. But
one wouldn't have to appeal to something outside the universe, to determine how the universe began. Instead, the way the universe started out at the Big Bang would be
determined by the state of the universe in imaginary time. Thus, the universe would be a completely self-contained system. It would not be determined by anything outside the physical universe, that we observe. "
Here's more:
"The conclusion of this lecture is that the universe has not existed forever. Rather,
the universe, and time itself, had a beginning in the Big Bang, about 15 billion years ago. The beginning of real time, would have been a singularity, at which the laws of physics would have broken down. Nevertheless, the way the universe began would have been
determined by the laws of physics, if the universe satisfied the no boundary condition."
http://www.hawking.org.uk/the-beginning-of-time.html
What Hawking is doing here is ruling out an immaterial cause a priori.
In summary.
1. Hawking claims that the universe and time itself had a definite beginning. (p2)
2. He apparently feels that the universe
had to have a cause (p1),
but believes that
it had to be a material cause, so he interjects imaginary time in his model in order to connect the material effect of the beginning of the universe to a material cause.