A homeless guy who sleeps under a bridge and uses it as shelter is not shelterless either if one goes by the literal meaning of the word. But the comparison here is really between homelessness and sexlessness.
Suppose you were homeless in a country where the outside temperature never drops too low. Even without access to a homeless shelter, you wouldn't die or experience rapid physical decline, provided you were able to obtain sufficient food and water. The stress caused by homelessness would likely reduce your life expectancy, but it wouldn't kill you directly. Just like inceldom, it would just make you miserable.
Some homeless people are better able to cope with the stress of living on the streets than others. Similarly, some incels are better able to cope with the stress of being sexless than others, but both inceldom and homelessness generally cause chronic stress, and that stress will result in a lower life expectancy.
A homeless guy living under a bridge may or may not be able to slightly alleviate his mental suffering through therapy, but I don't think he would be able to overcome it without escaping homeless. The same analogy holds true for incels. The only way for incels to significantly improve their mental state is by ascending.
Men who suffer from either objective or subjective social isolation are at a significantly
increased risk of premature death and disease. Inceldom is subjective social isolation as we are men who feel lonely because we are unable to attract sexual partners. Men who are married or cohabiting have a
much lower risk of disease than men who are single or divorced.
One could argue that humans need sex and intimacy as much as they need nutritious food or fresh air. If you never breath in fresh air or eat healthy food, you won't die immediately, but your life expectancy will be reduced. The same thing is true about inceldom. It won't kill you right away, but it will reduce both your quality of life and your life expectancy.