Have you checked that they support the C3 runtime? To see that the plugins are C2 only, create a little test project, and set it to use the old C2 runtime. Try adding those plugins. If you can, you’ve demonstrated that the plugins are working, but are C2-only.
I am sure they are C3 plugins. The Rex Tag text I got from his official GitHub and, as I said, I can see these plugins in the Addon Manager window. They are there with all the respective information. I just cannot include them on my projects because they are not in the "Create new object type" window.