The original author decided to put the argument in the next paragraphs:
The people of most countries will become very determined to fight back after the country is invaded and occupied, which is why much weaker powers like Afghanistan and Vietnam tend to prevail after being invaded and occupied even by great powers. We can expect the same determination after a nuclear attack—and yes, there would have been enough survivors in the USSR to continue the fight. Analysis by the US government in the early 1980s (again when IIRC nuclear stockpiles were at their greatest number) estimated that a full nuclear attack on the USSR would kill only about 55% of the population even if the USSR had no warning of the attack. The number for a full attack on the US was a little lower (50%) because the population of the US is more spread out as opposed to concentrated in cities.
Yes, the people most useful to advancing a Soviet nuclear program would preferentially have been in the 55% that die (especially if Washington attacks mid-week, when fewer of the Soviet upper class would be at their dachas) but Moscow could have used the surviving nuclear scientists to teach new nuclear scientists (and this time required them to live somewhere other the the probable targets of the next US nuclear attack).
The page you link to is silent on whether Von Neumann believed the US would have been able to keep the USSR from obtaining nukes indefinitely or whether the attack he proposed was intended merely to slow their nuclear program down. If the former, I bet no one in the Pentagon took his proposal seriously for more than a few days: the Pentagon would know that to have a realistic chance of keeping the USSR from obtaining nukes indefinitely, the US and its allies would have needed to permanently occupy Moscow and all of the USSR’s ports (after nuking the USSR of course) the success of which would have been in severe doubt, and even if successful would probably have caused the deaths of many millions of men on the US side.
The original author decided to put the argument in the next paragraphs: