I want to zoom in on this for a second. Now, I'm not an expert by any means, but actually... most of the people I know who do world-building actually wouldn't have a town of mostly humans and tolkienesque creatures. In fact, generally, the issue I've seen people run into is more "but there are not rules for symbiotic slime people and elves that are living trees" than it ever is more of the Tolkien-land.
This isn't to say it doesn't happen, but I do wonder if many of the new players I've seen and tried to help become DMs would even blink at that party comp, let alone need a section of the chapter to explain to them the options of allowing it or not.
True, I do hope the chapter encourages DMs to allow their players to help with the creation of the world. There are many benefits to that approach.
Honestly, I NEED to do top-down worldbuilding, because I need to know what people believe in. A world where people believe that all sentient life was created when the Sun cast the Moon into the Sea in an attempt to drown them is going to act, react, and talk differently than a world where they believe that Kings can gain power and ascend to be Gods if they control enough land and people.
I'm not against anyone taking any approach, I just know my approach starts far more at the broad picture, before usually making up the small details at the table.