Kernel module is basically the only way to implement this type of security software. That’s the only thing that has system wide access to realtime filesystem and network events.
Yes, they’re ultimately liable to their customers because that’s how liability works, but it’s really hard to argue that they’re at fault for picking a standard piece of software from a leading vendor that functions roughly the same as every piece of software in this space for every platform functions, which then bypassed all configurations they could make to control updates, grabbed a corrupted update and crashed the computer.
It’s like saying it’s the drivers fault the brakes on their Toyota failed and they crashed into someone. Yes, they crashed and so their insurance is going to have to cover it, but you don’t get angry at the driver for purchasing a common car in good condition and having it break in a way they can’t control.
What mitigations should they have had? All computer systems are mostly third party tools. Your OS is a third party tool. Your programming language is a third party tool. Webserver, database, loadbalancer, caching server: all third party tools. Hardware drivers? Usually third party, but USB has made a lot of things more generic.
If your package manager decides to ignore your configuration and update your kernel to something mangled and reboot, your computer is going to crash and it’ll stay down until you can get in there to tell it to stop booting the mangled kernel.