From my understanding of the question, you are trying to find the exact code which is used to perform the bicubic interpolation in “imrotate” function of MATLAB. “imrotate” uses the same algorithm as followed by the paper in the MATLAB Answer link that you have attached:
R. Keys, "Cubic convolution interpolation for digital image processing," in IEEE Transactions on Acoustics, Speech, and Signal Processing, vol. 29, no. 6, pp. 1153-1160, December 1981, doi: 10.1109/TASSP.1981.1163711.
Also, as a sidenote, “imrotate” already supports GPU Code generation and you can directly use that for your application.
Hope this helps!