The OP literally asked to "attack the ideas, not the man", and here you go attacking the ideas solely because of the kind of men who share it...

What do you mean by " it reads like something /pol/ or reddit would write "? Does Kaczynsky use tired cartoon memes? Does he accuse the left of insufficient weightlifting? Does he challenge them to a 1v1 in a Nintendo fighting game?

What part of his criticism do you actually find wrong?

Lol had not heard this one. The left has "insufficient weightlifting". Might be true.

Could it be that on the left there are more endurance sports, for health. And on the right there are more muscle building sports, so they can strike cool poses with their guns?

It's pretty common. Don't be surprised if you get called a dyel (DYEL, Do You Even Lift) lanklet (small, not fat or muscular) runcell (someone who runs to try to get women, but would be better off lifting).

That's overly reductive. Kaczynski's political views continued to evolve while in prison; his later writings are notably different in tone.

Industrial Society and its Future is the only work anyone ever mentions, especially where Kaczynsci's anti-leftist politics are concerned.

That's unfortunate because he expanded on ISAIF at length in his prison writing and took a more thoughtful and look at a lot of existing political tropes. For example, in his Anti-Tech Revolution* he lauds the tactics of early feminists' campaigns to obtain voting rights for women, and similarly deconstructs a lot of flaws in right wing ideology.

I disagree with a lot of Kaczynski's ideas and methods, but I still regard him as an important thinker and wish people would read his output in full rather than rushing to have the hottest take (not meant as a dig at you, rather the avalanche of attempted zingers on Twitter and in the media over the weekend).

His criticisms of leftist activism in the 1990s is that it will always seek to control and exploit technology to implement their ideology so followers of his anti-tech movement should not rely on them. He then rants against 'green anarchy' claiming it is a kind of naive forest worshipping cult that also should not be included in whatever anti-tech movement. It is a small part of his overall manifesto on who you shouldn't trust to join a specific (terrorist) movement and likely came from his time in academia.

It's definitely not the standard fare you would find on those 2 sites you mentioned just a brief 'don't trust these activists they want the philosopher stone for themselves'.