Yes, it uses some object types that are part of it (air pockets, some room effects, certain decals, etc). Once there is a mod that includes these types of objects separately it will require that instead.

the watcher DLC is obligatory for this to work ?

This is a great mod :D

to clarify, every time I re-open the game, all the new rooms' map positions get reset, and trying to render a map after making changes to it without reloading the game first outputs a blank map image.

OH! also
This mod doesen't show up on the workshop by itself anymore
i think you might have hidden the wrong one

Devtools can only save a map by overwriting another, so wherever the map would save (your mods files for custom regions and mergedmods? for OE) you should create "dummy files" by copying map data from another region and use the previous method after that

Hope this helps :)

to clarify, the map comes out blank whenever I do this

Go into dev tools, select the map tab, click "cannon view", rearrange the new rooms, save the settings and click "render map",
I'm pretty sure the new map files will end up in "mergedmods" but i'm not certain
When you find them just copy them into your mods OE folder
Then restart your save and enter outer expanse again and the map should work

I'm not sure how to fix the map, but I think looking on the wiki might help

https://rainworldmodding.miraheze.org/wiki/Creating_A_Region#Map_Files