I think the term stepfather is used to explain the role/relationship he plays in the household in relation to the child(ren). So it's less about marriage and more about playing a parenting role in the family dynamic. The semantics don't seem to matter so much these days. Which in this case, sure maybe it's a little annoying but we all obviously know who they are referring to. But of course he is not legally a stepfather and would not legally be her parent.