"iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays." From the page you linked. It hasn't been possible w

"iMac (Retina 5K, 27-inch, Late 2014) and later iMac models can't be used as Target Display Mode displays." From the page you linked. It hasn't been possible with new iMac models since 2014. I'd expect the Pro to be the same. Personally I would love it if tablets and new iMacs had video input to enable use as external monitors but unfortunately not