Male trees were planted disproportionate to female trees in urban settings because they didn't shed messy seeds. The preponderance of male trees increases the amount of pollen exposure. Author asserts this leads to an epidemic of allergies/asthma. Author asserts this indirectly leads to cancer.
Author fails to apparently take into account that although we are exposed to more male trees than female trees, our total tree (and therefore, pollen) exposure is far lower than historic norms. If adverse consequences are due to pollen over-exposure, we would still be net underexposed to pollen, so any increase in adverse health consequences couldn't be explained by that.
Author also presents themselves as a "horticultural epidemiologist." Worth noting that a google scholar search suggests this person is not an academic and holds no academic credentials, but he does make his income flogging a pair of books on this topic. I guess "Epidemiologist" isn't a protected title, but I usually don't see it applied to someone that doesn't actually work in epidemiology, or hold a degree in epidemiology.