I don't want to spoil, but the optimism isn't for the characters, it's for we the reader, and the species.
The thimble of fire joins the wider flame. Goodness survives even there, and even then.
That fire they carry is not extinguished even in a world where all systems and pretenses at civilization have been ruined. It finds the wider flame, and decent folks to tend it.
It is one of the more optimistic works he's done, not despite the setting, but because.