I don't see how that link supports your assertion, nor really even the supports the general idea that docx isn't open. Their main complaint seems to be that it's a big standard which makes it hard to implement. Honestly it gets to be a bit hard to take these free software groups seriously when they move the goalposts and try to redefine what common words mean. It's an open format by any reasonable definition and no amount of whining by free software advocates and scare quotes changes that.