Evidence so far would suggest that it's closed source.

The archive downloaded by the installer script does contain some source code, but it's mostly "generic" TensorFlow code, with some Python stubs that call off to native libraries (as you'd expect). It seems like all of the ML Compute stuff is contained within pre-compiled libraries (with some header files provided), but no source code.

I could be wrong here, and it might be that the intention is to open source the ML Compute components, but I don't think that's been done yet.