Which means you're permitted to parse the files and handle them according to spec., which is notably different to handling them "exactly as Excel does" (e.g. in cases where the spec. allows implementation variance, which I believe are frequent with OpenXML formats), particularly where Google would be testing with Excel software in order to ape implementation specifics that are absent from the spec. That's the potentially worrying differentiator in this case.
The older specifications are available as well, I don't think they have any specific requirements about their use. I think they called it CDF back then.