HDMI Receiver Sense Logic and TMDS Capacitor Coupling
I don't know much about HDMI hot plug detection, but I guess all recent high-speed serial busses (DC de-coupled) use the the same technique as in USB 3.x standard. They use the idea that unterminated differential line has shorter RC time when charged by a pulse up (only parasitic line capacitance is present), while terminated line will have the RC close to AC decoupling capacitor. There is a picture from most recent USB 3.2 specifications, section 6.11.1:



See Section 6.11.2 for more details of the Rx detect sequence.