I am not sure why you are being downvoted. I have been doing a project with Homogeneous Transform Matrices recently and also assumed that this was "obvious" sin

I am not sure why you are being downvoted. I have been doing a project with Homogeneous Transform Matrices recently and also assumed that this was "obvious" since the shear diagonal lives in the rotation part of the matrix. Still, it's a very neat trick and I appreciate the OP for posting it!