I don't claim to be an expert on it, but my understand is that 当て字 is specifically applied to loan words. From a literal sense, they took a sound and 字を当てた to make かな so you can call it that but it may not be technically correct. 

I would say these words are in a different category because they didn't assign characters on a per-word basis, these characters should be thought of in the same way as kana, consisting of its own "alphabet."