Both HDMI and DisplayPort can use HDCP. The authors of the DisplayPort standard also invented their own clone of HDCP called DPCP, but you can also use HDCP over DP, and it seems like HDCP is a lot more popular(?). In any case the DRM implications (in terms of lockdown and code signing) are the same. Though not explicitly mentioned in the screenshots of the i.MX8M reference manual shown in the article, reading the rest of the manual it's clear DP is affected too. (Which, again, is why the MNT Reform forewent the use of the DP interface to attach their eDP internal display in favour of a MIPI DSI to eDP bridge.)

I certainly agree that restrictive code signing practices like these are completely unacceptable. In my view, the 'right to repair' movement should encompass a right to any keys needed to fix bugs, though I am not optimistic about 'right to repair' being interpreted in this way.

Worth noting that while we usually think of DRM as a (patently ineffective) way to prevent piracy, in reality it seems to be much more about controlling end devices and the manufacturers of end devices. I explain more about this here: https://news.ycombinator.com/item?id=17588610