I get your point, but I think you're misunderstanding the author's intention. He's not trying to build a legal case for prosecution and shared responsibility; he's arguing that political actors (especially on the right) work to exploit social grievances across networks to build coalitions. And, if we look at the last decade, there's PLENTY of examples:

• Steve Bannon understood Breitbart as a unifying right-wing project that could exploit online grievances. As he wrote to Milo: "Dude---we r in a global existentialist war where our enemy EXISTS in social media and u r jerking yourself off w/ marginalia!!!! U should be OWNING this conversation because u r everything they hate!!! Drop your toys, pick up your tools and go help save western civilization."
• The entire path of Gamergate from rightwing 4chan swamps into a pro-Trump organizing tool
• Hell, 4chan launched its /pol forum the same month that its founder met with Jeffrey Epstein

So it's not a question of "Are all these groups coordinating together?" They're not, and the author doesn't claim they are. And also, that's not really how political coordination works anymore. (Although, group chats among wealthy & influential right wingers are very real)

It works much more like, "Do all these groups understand themselves to be pursuing somewhat similar goals, and do they avoid intergroup warfare, and do political actors exploit their grievances and rally them around a single cause (like, say, anti-trans issues)?

And the answer is very much yes.