That fact than no one here knows the answer probably points most turning the blind eye to it.
I'm willing to think that is true.
Another thought is that it may be a bit like with USB. (
https://www.usb.org/logo-license )
Using the official USB logo implies you are licensed (and that your device passed compliance testing). But many manufacturers do not use the official logo and thus don't infringe on that AFAIK. The USB case is a bit trickier though as if you buy an USB vendor ID, you'll have to apply for USB-IF membership (if it hasn't changed), so you'll be granted the license without extra step except that your device is still supposed to be USB-IF compliant.
In the same vein, it's likely that selling a device having HDMI ports only requires licensing if you both use the official HDMI logo (
https://www.hdmi.org/logosphotos/logos.aspx ) and claim HDMI compliance.
Most sold USB devices are not really compliant (not provably at least) and can't claim USB compliance, but that doesn't prevent them from being sold, and I'd guess that the same holds with a lot of devices with HDMI ports.
Obviously the safe way is to directly ask the HDMI alliance:
https://www.hdmi.org/