This is "Open Hardware" which usually means open PCB or chip schematics, so people can modify or extend the board. OpenWRT is "Open Software that runs on closed hardware".

After checking a couple, Kind of seems like a lot of boards on this "open hardware" list might not actually be open hardware?

Here's an example of what open hardware is supposed to be: https://github.com/greatscottgadgets/ubertooth/tree/master/h...

Yeah, that OnePlus phone is defo the very opposite of open hardware...