RE set is not closed under complement. So, given a r.e. language we cannot say if its complement is r.e. or not.
But any regular language is also r.e. and complement of a regular language is regular and hence r.e. too. Here, for a particular subset of RE set we say complement is closed.
Similarly in the question, we do not have the general RE set but a subset where the languages are not recursive. For those languages complement is guaranteed not to be r.e. -> reason given in answer.