Figured it out with help from Gomorrah on Ashers discord, basically, I had to tick this box
to make it be Suggested. Then I can search for it when in the Niagara Module Script (call it NMS for short) node graph, make sure to untick “library only”, then it will show up.
The reason it doesnt let you drag it from the content browser directly into the NMS Node Graph is because a niagara function module is like… a component or something, and a function in a NMS Node Graph is another type of object which lets you select that component, and it becomes the function.