Thank you. Here's where my confusion comes in
"iMac (Mid 2011) and later: target Display Mode does not work with Mini DisplayPort cable"
When you attempt to connect an iMac (21.5-inch or 27-inch, Mid 2011) and later to another computer with a Mini DisplayPort to Mini DisplayPort cable, the screen of the target iMac will not become an external display when you press Command-F2 to activate Target Display Mode.
iMac (21.5-inch, Mid 2011) and iMac (27-inch, Mid 2011) and later computers support Target Display Mode via Thunderbolt to Thunderbolt cable (2 m) when the source is another Thunderbolt-equipped computer.
iMac (27-inch, Mid 2011) computers do not support Target Display Mode via Mini DisplayPort cables.