What would happen in regards to a pre existing condition like type 1 diabetes?
If the patient has been paying for health insurance since before the diagnosis then the health insurance will pay as usual. Where the health insurance has limits is things like someone without any insurance gets diagnosed with cancer, they then won't be able to get easy affordable insurance to treat that and will have to look at other options.
It is like car insurance. You can't crash your car and then Pay for car insurance and expect the insurer to pay for a new car. Insurance works by making regular small payments before the incident.