whatever game or sport you are partaking in you are always learning, always improving your game. So even though the rules of draughts are a lot more simpler than chess both games involve outwitting another human player and then you have a whole other bunch of skills that you need to develop outside of simply learning the rules of the game.
the term "skill cap" or "skill ceiling" is only used when talking about video games. Do you know why that is? it's just a bullshit term used to poorly legitimise their choice of game.
so what if the game is easy to play? do you have fun playing it and can you easily get people to play against? that's all that really matters and the quicker people can accept it the better.
Skills like perception of opponents demeanor, or knowing when to do something unexpected probably don't have a skill cap (for some people). However, doing something in an RTS such as group selecting units and telling them to do things has an arbitrary limit to how well it can be done. Not the decision making behind the move, but how good you can ultimately be while performing the physical actions necessary. Saying "skill cap isn't a real thing because you can always get better at something" is semantics. It's referencing the minimum mechanical requirement to perform all the tasks at the best possible speed.
As far as your conclusion re: "skill cap" only being used in reference to video games... Every word has its origins.
Don't get me wrong, I like CoH's pace. But it would be a really useful feature in custom games. If I want to try a build 5 times vs someone in a custom, it'd be nice to put it to x4 until the pio/engie battles starts.