Even at the more generous level of “mental illness is different from physical illness” it is wrong, and dangerously so.
> The word "illness" should be used to represent a cause-- and resolving the cause should cure the illness.
This has never been what “illness” means; illness (both in general and in specific named instances) is often recognized and identified long before causes.
> The conditions which we call mental illness don't have a knowable cause.
They don't usually have a known cause. That they don't have knowable cause is an unwarranted conclusion. Plenty of physical illness don't have known mechanisms, and most of those that do were noticed and given names as illnesses before (sometimes centuries before) the mechanism was identified. This doesn't mean they don't generally have knowable causes, as is evident from the ones whose causes were later learned.
> Illnesses should be hypothetically curable.
That doesn't even remotely follow. Having a know cause doesn't imply curability because causes are not necessarily reversible.
> I think often the hacker news crowd forgets how ill-informed most of the world is
True. It's easy to forget that many people do not even realize that pattern matching on symptoms is how much diagnosis of physical illness is done, and that even where causes are known it's often not worthwhile (and sometimes not practical) to actually test for them directly.