Can't speak as to whether they have become more selective or to who they're targeting these days, but as a parent who has been there a while maybe I can clarify a little. It is a middle school and high school, in that students take classes for either middle school or high school credit (or dual enrollment high school/college credit for courses taken at UNR). The content of the DA classes, though, tends to be kicked up a notch no matter their designation. So I think, maybe, they're trying to say that a DA middle school class may more closely resemble a freshman or sophomore high school class in a typical high school, in terms of curriculum and the skills expected of students. That said, if it is designated as a middle school class, students receive middle school credit, even though the class may be accelerated by "regular" school standards. So, no matter what age, maybe incoming students need to have completed 5th grade and be ready for the DA version of 6th grade, which may be considerably more rigorous than a typical 6th grade experience. Anyway, that's what it seems like, but, of course, the admissions folks can answer these types of questions best.
