The post reads as a series of criticisms about mongo. I don't love mongo, but I'm not aware of any data store that can perform that type of query purely from an index.
Now, the description was vague enough that he could have been describing a real bug I'm not aware of - at one point I've seen MySQL decide to use an index for sorting instead of for filtering when that query plan was 500x slower. If mongo has a bug like that one, disregard my comment please. :-)
