TL/DR: the book is wrong.
What am I missing?
Nothing, as far as I can see. Even the erratum version presented in comments …
A pointer to void will never be equal to another pointer to void.
… simply is not supported by the C language specification. To the extent that the author is relying on the language specification, the relevant text would be paragraph 6.5.9/6:
Two pointers compare equal if and only if both are null pointers, both
are pointers to the same object (including a pointer to an object and
a subobject at its beginning) or function, both are pointers to one
past the last element of the same array object, or one is a pointer to
one past the end of one array object and the other is a pointer to the
start of a different array object that happens to immediately follow
the first array object in the address space.
void is an object type, albeit an “incomplete” one. Pointers to void that are valid and non-null are pointers to objects, and they compare equal to each other under the conditions expressed by the specification. The usual way that such pointers are obtained is by converting an object pointer of a different (pointer) type to void *. The result of such a conversion still points to the same object that the original pointer did.
My best guess is that the book misinterprets the spec to indicate that pointers to void should not be interpreted as pointers to objects. Although there are special cases that apply only to pointers to void, that does not imply that general provisions applying to object pointers do not also apply to void pointers.