Probably just my tone, or felt unpleasant at the idea that they’ve integrated fascist ideas into their worldviews.

Nope! I’ll check it out.

You can just Google “overpopulation ecofascism” and the first few results in normal and scholar mode are pretty good (the former for well cited intros, the latter for more in depth stuff on the topic)

Any ideology that incorporates vying for genocide is pretty close to fascism.