Specifying with world_nether works but the_nether doesn't.

When trying to specify the world selection to the nether, the_nether specifier does not work.

The Pregeneration wiki states that you can use world_nether or the_nether but only world_nether works. Maybe this was intentional and the wiki never got an update.

Edit:

I also tried the other optional specifiers (overworld and the_end) and those didn't work either.

Hi, the wiki looks correct. Read the note above the given world list, you will see that the name depends on the version of chunky you have installed. As in, the or there is exclusive not inclusive.

In your case, you probably have the Bukkit platform version installed and so only the Bukkit world names will work. On the modded platforms such as Fabric and Forge you would use the vanilla dimension ids (the ones that aren't working for you).