The insurance company cannot simply deny coverage based on an arbitrary advisement. Some doctors advise don't drive for two months, some say three, some say six weeks. It varies. At my five week office visit, I asked if I could drive home, and he said, "Sure, just be careful."
I am not suggesting one should try to drive when they are incapable of controlling their vehicle. That would be negligent and possibly border on criminal. However, I am suggesting that individuals recover at different rates, and it is normal to expect that some people will be fully capable of driving before others.
Refusal to cover an accident claim would first require proof that you were at fault; then it would require proof that you were incapacitated and could not control your motor vehicle. It would then require further proof that the inability to control a motor vehicle is directly related to the surgery, and that you, as the driver, disregarded specific instructions from your medical caregiver.
A cautionary warning in a informational bulletin does not constitute specific individual medical instructions. If that were the case, auto insurance would not cover half the people in this country who are on medications that have similar warnings on their labels.