"Likewise, the ground connections (AGND, DGND) should be kept separate as far back to the source as possible (i.e., separate the ground planes on a localized board, even if the grounds connect to a common point in the system)."
Hmm, thats interesting, Ok, I guess I see your confusion, my comment was from my experence with the datasheets I have read, which is only a small percentage of the available parts, so I spoke to hastily.
Relooking at some of Bryant's lecture notes (the author of the reference article) I noticed he did comment on data sheet writers FWIW
Bryant quote "Most ADCs have separate analog ground (AGnd) and digital ground (DGnd) pins, but too many engineers, and, unfortunately, too many data sheet writers as well, are uncertain how they should be connected. The lecture considers the nature of the currents flowing in these pins, the vulnerabilities of precision data converters to internal and external noise, and the effects of various grounding and decoupling configurations, and suggests, and justifies, a grounding scheme which gives the best possible converter performance in the vast majority of cases."
that lecture is here
formatting link
In any case I wouldn't second guess the datasheet writer, I would ask Analog Devices for clarification on this point for this particular part, there are always exceptions to the rule, and errors in the datasheet, you have to found out which it is.