Regardless, it is a bad decision by the homeless person and an example which a better informed author would have left out. Insomnia is less harmful than hypothermia.
It isn't that alcohol doesn't really warm you, it is worse than that. It actually makes your body core colder while giving the illusion of the opposite.
There is more to it than temperature that is overlooked by brevity.
When you're in a stressful, and uncomfortable, situation like that the numbness helps with the cold, possibly uneven, ground.
It can quiet the mind. And the inhibition can help nullify the fight or flight response aiding in getting to sleep.
There are several properties of alcohol that makes it seem like a reasonable deduction to drink a bit in order to sleep. Even if it is harmful and could compound the suffering down the line.