The author is wrong and right. Stress is subjective. True. There are no standard tests that can validate a person’s claim, or at least ones that are being used this way. We go by what people say. This leaves the door open to fraud.
He is wrong that stress is never a disability. It certainly can be. It can lead to mental break downs and physical health problems. So his example of Steve Jobs is ironic. Stress could very well be what killed him since stress definitely lowers your immunity and ability of your body to fight illness.
This is the same reason you should never trust a person or politician who says they're for restrictions on abortions with the exception of personal or "mental" well-being of the mother. That give them wiggle room to always find that having a baby will cause the mother stress, or some post-labor depression.