Agreed. The Github code is a substitute for the take home assignment. If his GH code is good enough as per the employer standards, invite him in office to explain the work he did on that project.

Now employers don't want to do that most likely I assume is because no one wants to go through the pain of figuring out the details of GH code. OTOH, the employer is expecting the candidate to understand the problem assigned to him and come up with a solution!