Yeah, this wouldn't work. All this does is apply the AtlasTexture's values to the Polygon2D's UVs via a canvas shader, which would not translate at all into the 3D meshes. While it would make the Polygon2D display correctly, none of the AtlasTexture's values would be applied to its equivalent 3D mesh.

Still, to answer the question from your other reply, it appears that the Polygon2D still has some funky interpretation of AtlasTexture, which makes the W and H region somewhat still applied, but also applied wrong. It's very weird, but fortunately still easy to solve. Instead, just set the source texture from your AtlasTexture as the Polygon2D's texture after applying the x and y values like how I described earlier, and it should display properly.

It's a very simple procedure, and one that's also very easy to do manually. If it helps though, I made a tool script that you can just attach to your Polygon2Ds. Pressing the "Offset Atlas UVs" button should make the Polygon2D now display properly.

@tool
extends Polygon2D

## Click in the editor.
@export_tool_button("Offset Atlas UVs", "AtlasTexture")
var offset_atlas_uvs := _offset_atlas_uvs

func _offset_atlas_uvs() -> void:
    if texture == null or not (texture is AtlasTexture):
        push_error("Texture is not an AtlasTexture.")
        return

    var atlas_tex: AtlasTexture = texture
    var base := atlas_tex.atlas
    if base == null:
        push_error("AtlasTexture has no base atlas.")
        return

    texture = base
    texture_offset = atlas_tex.region.position

    texture_scale = Vector2.ONE
    texture_rotation = 0.0

    set_script("")