It's a boot argument to the kernel, stored in NVRAM. These arguments are normally mutable. Apple had to write code to prevent modifying said arguments. Said code can have flaws.

But lets say you don't find a vulnerability in SIP userland detection, and instead find a kernel exploit to get around the protection:

If malware were to figure a way around it, then even antivirus software can't uninstall it. Only Apple can. It's not FUD.