Part of what you're paying for is the right to use the trademarked tern HDMI, just like how the USB Consortium charges you stupid money to use the USB logo.

The suit over usage of "HDMI" in a reverse engineered version would wind up arguing whether or not HDMI is a genericised term and the HDMI Forum would lose their trademark. They will throw every cent they have into preventing such a decision and it'll get ugly

Can't you use a trademark to refer to the thing as long as it's clear you're not claiming to be them? Like if you say your PC is "IBM compatible" you're not claiming to be IBM, are you?