This is one of the several submissions in MATLAB File Exchange on MATLAB Central which is a forum for our product users to interact, exchange information and knowledge, without MathWorks' involvement. You may contact the author of this submission directly for specific questions about the implementation.
However, you can obtain the same results using the following function:
Also, if you still need a way to convert hexstring to double precision number then you can consider using hex2num: https://www.mathworks.com/help/matlab/ref/hex2num.html
Hope that helps!