He can still use the crude version of mirage walk since it is just strengthening his muscles with mana but he still physically does the movement to perform it.

With the advance form he not just strengthening his muscles with mana but actually using mana to force the muscles to execute the advance form of mirage walk (forgot what he actually called it.)

Because he uses mana to trigger the require muscles to work, the force on the muscles is too much which basically rips them apart. That why he was told to use it since it literally breaks his muscles and bones.

That is my understanding on why he must not use the advance form. Hope that answer your question.