Anti-cheat software is very clearly and explicitly spyware. That’s the entire purpose of it. It spies on how you use your software in the hope that if you cheat you’ll be seen by the spyware watching you.
This spyware is generally not something people want on their computer - as evidenced by people complaining about it. So effectively whats happening is that people are being spied on against their wishes. Spyware is a common category of malware.
So I think it’s pretty easy to see why people might describe anti-cheat software as malware.
[edit] I’d posted something to go into more detail. But I’ve decided that branch of conversation is not really the way forward.
I’ll just say that the software is not installed by choice, and it does things that people don’t want it to do… so it could be described as malware. But if you want it on your computer, then I guess for you it is not malware. In any case, it doesn’t look like we’re going to agree about this regardless.