Poor them, who hate chocolates.
The who-clause is not an appositive. An appositive is a description of a noun, that works with the noun. For example:
Chocolates, a sweet delight.
In this case, a sweet delight renames the adjacent noun and so is an appositive.
Poor them, who hate chocolates.
The structure of this sentence can be explained through Subject Complement Agreement
The Subject Complement follows a linking verb. For example:
Poor me, hating chocolate
I am the subject and "chocolate" follows a linking verb (to hate) and so is the subject compliment.
The phrase in question is Subject Complement Agreement
Poor them, who hate chocolates.
Subject Complement Agreement comes into play when plurals are involved, either the subject is plural or the subject compliment is plural or both are plural.
The confusion comes when you apply a plural compliment to a plural subject, do both subjects hate both compliments? Does one subject hate one compliment and vice versa? The answer is that this phrase is acceptable but ambiguous. A better phrase would make it clear exactly which subject hates exactly which complement. For example:
Poor them, who both unanimously hate all chocolates
Poor them, who each hate a specific flavor of chocolate