No it doesn't. It supports a transitional, half-baked, non-standard implementation that is incompatible with the ISO standard. It's sort of like MS Office's support for ODF, which is also non-standard. Microsoft do not support standards (even ones that they push for), they re-invent them.
MS doesn't use DOCX internally because of the rendering bugs that they can't purge. The faults of DOC format are at least known and quantified, and people have worked their way around them. DOCX, I know of exactly zero benefits to using it...