Agreed that all-zero utility function is more or less just wrong.

People like this can still remember what happiness is and wish that they were happy; they can dislike feeling nihilistic.

They can still experience all sorts of things as unpleasant, such as making an effort.

A state of mind in which happiness is very difficult to obtain and drive/motivation is at an extremely low ebb is not a zero utility function.

Nonetheless I find it very easy to understand why "zero utility function" would be used in this case as a poetic metaphor.