But technically the Thunderbolt 2 connector's the same as a mini display port right? "Thunderbolt is fully compatible with Mini DisplayPort peripherals . Technically, the plug is the same and the only difference is that the Thunderboltconnector can carry – in addition to the classic Mini DisplayPort – a Thunderbolt signal, which is a signal PCI-Express (4 lines)."
You would need to enter target display mode which is simply not supported on machines after late 2014. Additionally target display mode was very picky and did not support hdmi to mini display port or thunderbolt.