It's an apples to apples comparison of "what hosting software is used across all domains", not "what hosting software is used across all domains in the space Microsoft competes for".
That may not be a particularly useful metric in business terms, but it is at least a fair comparison.
People create these stats for a reason. If the reason is to show that one side or the other is "losing", whatever that means, they should go ahead and chose whatever they want. (I am not implying that's what you were saying or anything).
On the other hand, if the point is to educate people on which software is more used in a meaningful sense, the article would be better if it had the grandparent's explanation at the top, explaining why the comparison should be done only in one specific market, and then do the comparison in that market. I for one wouldn't have thought about it, and gotten the wrong impression.
That may not be a particularly useful metric in business terms, but it is at least a fair comparison.