They are all linear under a given illuminant: double the input, double the output. The fact that they don’t behave like CMFs is a different issue, no?
Yes, you are right, they are two separate issues. One issue is that we talk about linearity with respect to imput intensity, holding everything else equal. The other issue is whether there is a linear mapping from the camera “RGB” values into a standardised colour space. In the former case we have linearity (in some sense), in the other case we do not.