PSA: Don't tell Western doctors/therapists that you're a Leftist. The American Psychological Association (APA) classifies anti-capitalists, animal rights advocates and environmentalists as terrorists.

They're talking about the people that spike trees so lumber workers may get injured, or burn down labs where animals are being tested on.

I'm pretty sure they're talking about people engaging in direct action like fighting with / firing on the police in opposition of an oil pipeline, or destroying GMO crops. The wording of the article is bad; I don't think anyone in their right mind would consider donating to an animal hospital or picking up trash in the park to be extremism.