Well, the problem isn't so much that some exotics are unattainable as it is that no one told us they were unattainable. So we all wasted our time and effort trying to get them. Unaware it was impossible. Bungie specifically put in place a new bit of code that lets them keep items *out* of the database such as Sleeper. But they didn't do that with these and the armory specifically says they can drop in TTK. Not that they are quest rewards or anything.