Why do TQFP and TFQN have different advice regarding analog ground?
In general, datasheets give excellent advice that you ignore at your own peril.
(Alas, like the advice of Cassandra, the advice in datasheets is often correct but misunderstood until it is too late).

However, even the writers of datasheets occasionally make mistakes.

The Burr-Brown application bulletin "Analog-to-Digital Converter Grounding Practices Affect System Performance" reports one experiment where a PCB was layed out both ways, and shows "a single ground plane" works best, even when it contradicts the "split grounds" advice given in a datasheet.

Splitting ground planes often seems to make an improvement.
Making a single ground plane and fixing the actual source of the problem is even better.
The Henry Ott article you linked and other "unsplit ground" articles at the MassMind explain why.