Seeing in MATLAB code how ode45 is implemented != how the thing is running on the machine. That's a very small top slice.

But okay -- as I mentioned, you're entitled to your views!

fft.m is the more obvious example of the closed source algorithm here. You open it and it just says

% Built-in function.

The algorithms written in C and compiled by mex are the "built-in" ones that are not viewable.