Possibly you do the subdivisions along the edges uniformly in the target space, and map them to uniform subdivisions in the source space, but that's not correct.
edit:
Comparison of the article's and the correct perspective transform:
Completely backwards. Math is much more difficult than programming and LLMs still can't consistently add numbers correctly last I checked. What a strange attitude to take.
It’s fascinating how plausible it looks at a glance while being so glaringly wrong once you look at it more closely.
Edit: I think there’s a lot of confusion because the edges of the cube (the black lines), do not incorporate the perspective transform all along their edge. The texture is likely correct given the focal length, and the cube’s edge is misleadingly straight. My bad, the technique is valid, but the black lines of the cube’s edge are misleadingly straight (they are not rendered the same way as the texture)
I opened an issue ticket on the repository with a simple suggested fix and a comparison image.