Zero-days are not "back doors".
Unless the zero day flaw was put there intentionally, as back doors are put there intentionally, a zero day flaw is not a back door, it's just some incompetent who should be employed asking me "Do you want fries with that?", rather than employed writing security sensitive software. In other words: your average bad programmer.
I think the implication of the story is that they are put in there intentionally, at least some of them.