The post is essentially reminding people that XML doesn’t magically equal openness. A schema can be “unnecessarily complex, bloated, convoluted and difficult to implement”, and in the case of Office 365 the spec runs to “over 8 000 pages” and uses deeply nested tags, overloaded elements and wildcards. The result is that only the vendor can feasibly implement it, which eliminates third‑party implementations and lets t…
Considering how little most free software makes they can't afford to do a lot of things. It's not a hard bar to hit.