Thank you for providing the scene. It helped us identify where the issue is. To start with, let's look at how Texel Density is calculated. If we take a single polygon and compute its TD using the classic formula:

UV Area / Mesh Area = TD value

Since our task is to calculate this for an entire island, we first calculate the average UV Area across all polygons of the island:

(face1_uv_area + face2_uv_area + face3_uv_area...) / island_face_count = UV Area

Now that it's clear how this works, let's return to your geometry. In the following image, I created a copy of your larger rectangle and unwrapped it from scratch: