Um, yeah, and considering that the AZW3-calibre format, if you'd like to call it that, is fully compatible, your point is???
Let me point out the obvious to you, which I have already stated: a fully reverse-engineered proprietary format is a rather hollow claim. For all intents and purposes,
besides for people who wish to sit in a circle and meditate on the nature of words, it is not a closed system.
The concept behind making a proprietary format is an attempt to prevent others from making their own tools to deal with such formats. Sometimes, they fail.
There is a legal and aboveboard reverse-engineered implementation of MOBI/AZW3 (actually more than one) which is an open format spec, and thus my AZW3-calibre files (byte-identical to your "pure" AZW3 files) are an open format.
I could just as easily say that some ZIP implementations are closed, therefore EPUB which is based off of ZIP is a closed format.