I have the same question. I thought hem/hun referred to people. Which kinda makes it a strange statement. No weirder than "ik ben een appel" i suppose.
As from what I understand, 'ze' is the unstressed form of 'them', which is also used to refer to objects. 'hen' is used when 'them' is considered as a direct object', while 'hun' for indirect object.
It should be accepted, unless it was a type-what-you-hear exercise (because the pronunciation of 'jij' and 'je' is different).