I actually do have a lot of exposure to patents, and I know everyone uses DocX already. I'm just saying that that web page doesn't say why they chose DocX, only why you should use DocX.
It seems more like rationalization than reason.
Interestingly, a section in the wiki article linked mentioned the standard proposal was controversial because ODF already existed (and ODF was considered less complicated as a specification).
Nevertheless, good point. It depends on what you mean by "open".