I strongly disagree with that, there is a big difference between forest and woodland for example, while the later describe an area covered in trees the former describe an area that is covered in trees and undergrowth and is generally harder to move through.
Stronghold and urban are also two very different things, outsider is meh IMO and don't let me start on merging Fey and Giant into Humanoids.
Obviously there are lots of distinctions that could be made. I mean, the alpine forests of northern Canada are pretty darned different from the forests of southern California. The question is, is it a distinction big enough to justify putting it at the "top level?" To have separate categories for Woodland and Forest implies that they are as different from one another as Forest and Desert, which is silly.
[Edit: I misunderstood Stronghold originally--I thought it was "castles or mountains, inhabited by dwarves" where it's actually "castles, or mountains inhabited by dwarves." I still think it's silly, just for different reasons...]
As for Stronghold, are we really going to have a whole terrain type just for castles? What's next, a Tavern terrain type where you find adventurers, and a Church terrain type inhabited by clerics? What is the practical use of a Stronghold terrain? Other than dwarves, what monsters are you going to find there?
Keywords were invented to handle just this situation--they allow for fine-grained distinctions within larger categories. If Celestial, Elemental, and Fiend are three different categories, then any time you have a spell that does something with extraplanar entities, you have to spell out "celestials, elementals, and fiends." And then some designer pipes up and asks whether slaad are considered fiends or elementals or what, and you end up having to add a new category just to cover slaad, or shoehorn them in someplace they really don't belong, and it's just a mess.
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