Microsoft Office do not produce or read Office Open XML files as defined in the OOXML ISO standard. They produce something that looks similar but not quite right. The same thing happens with the older formats, they are not produced or read in the way it is written in the published specifications.
The same thing also happens with OpenOffice/LibreOffice and the ISO ODF spec. In practice, this is not a big problem with either OpenOffice/LibreOffice and ODF, or Microsoft Office and OOXML, because the deviations from the spec are minor. For instance, almost all the deviations in Office from OOXML transitional are due to last minute changes in the values of some attributes.
BTW, don't cite techrights.org. They are one of the most unreliable sources of information on the net.
The binary formats are open too. Mind you, both OOXML and the binary formats are quite tightly coupled to Office's inner workings and implementing them is probably not easy either. But documented they are.