More pollen may translate to more fruits/seeds, which may translate to more saplings.

The first step would require a corresponding increase in pollinators, but pollinators (mostly insects) have reportedly been declining for decades.

I’m curious about the “amb a 1 allergen” which has been increasing according to a link in the article. Do we know what role it plays for the ragweed in reproduction, if any? What’s the advantage here?

Correlation is not causation. More co2 and favorable conditions lead to more vegetation. Large ecologies may have dynamic mechanisms that tend toward homeostasis, but the systems are chaotic and fundamentally unpredictable. That climate change produces one result or another is a secondary consequence of complex systems interacting. There is no earth thing "fighting" climate change.