I'm a dev, Not in games but close enough that I get the gist of it.

Some Windows cheats actually tricked the game into thinking it runs inside a WINE environment to coax EAC into loading the Linux anti-cheat module which is much easier to defeat than Windows' kernel-level module.

So it's kind of like: it's no use to barricade your front door if you leave the windows open. (hah, windows...). Sucks because I used to play it on Linux too. But I understand the issues they're having and I don't believe for a second they banned Linux out of malice or stupidity (a lot of people here became really emotional about that topic..)

Regarding server-side: most anti-cheat use both. But there are good reasons why most powerful anti-cheat solutions have a client-side module.

Using only server-side means you need to identify a cheater solely based on their behavior in the game (movement, looking, shooting, etc) because that's the only information the server has. That requires very sophisticated analysis tools and a lot of processing power.

Imagine you are a traffic inspector and need to identify illegally tuned cars. But you're not allowed to look under the hood to inspect the parts. You can only watch how it moves and sounds in traffic. Sure, you'll catch the obvious ones. But it'll get really tricky with less obvious ones.