You cannot grab an iMac, plug it into a silicon Mac, and get a picture on the iMac (without 3rd party software). This is incorrect. Please go look this up. The feature stopped working when silicon Mac’s came out. It only works on Intel Macs. 5K iMac or not.
Target display mode, the feature that allowed this, is no longer supported.
Target display mode, the feature that allowed this, is no longer supported.