Physically the port may be the same. Beyond that, they are not. Even the cables' internal wiring is vastly different. One is a technological advancement of DVI and the other is an external PCI-E bus. Not the same at all.
Many people, thousands of them, would be very interested how you got a video connection from a non-Tunderbolt mini-DisplayPort source going to a Thunderbolt 27" iMac. Even Apple engineers would be very interested, because it cannot be done.
I have no quarrel over if an Xbox/PlayStation or a generic Dell/HP machine can work as a video source for Target Display Mode when the iMac that is used as the display is a 2009 or 2010 non-Thunderbolt equipped 27" model. The problem is within your logic that Thunderbolt and mini-DisplayPort are the same when they are vastly and nearly incompatible with each other. A metaphor would be like comparing USB 2 and FireWire as being the same when they are not. A 2011+ Thunderbolt equipped iMac simply cannot work in Target Display Mode for a non-Thunderbolt Mac.
See item 4 at from Apple:
http://support.apple.com/kb/ht3924
Of note, both of those videos were posted to YouTube before the Thuderbolt equipped iMac was released, rendering them as irrelevant to this discussion and serving to only further my case in point.