The Population Bomb was a political call to arms that used science to give it credence.
It turns out that much of the science was really, badly flawed. And not in subtle ways.
Revisited, while acknowledging some of the egregious issues, tries to diminish them by saying other experts thought so too or something else went bad so the point is still valid. It then goes on to say that they were still kind of right and it'll all go pear shaped in the future (they're a bit more careful not to say when this time).
But that's not science. Their conclusion was based on their predictions and their predictions were wrong. So their conclusions, even if accidentally correct (which they don't appear to be), are not justified by the evidence.
I find this lack of respect for science, by a Professor of Biology, quite astonishing.
He would have taught his grandchildren to carry the message on, but he already knows they are doomed.