OpenRA

Open in regard to software development means "Open Source". As a game, OpenRA requires proprietary software just to work. The title misleads users into thinking that the game is Open Source, when in fact, it's not.
To make an analogy, that's like selling a car, claiming it is rust free(in this case open), apart from the rust in the wheel arches(the Freeware bits needed to make the game actually run). Then it's not rust free is it.

The developers clarify this in the Legal section of the site, but the title of the game is very misleading, and not everyone reads the Legal section before downloading this game, and several gaming sites online list this game as Open Source, which in turn leads more people into thinking this game is Open Source.

The source is open: https://github.com/OpenRA/OpenRA/
That you need the assets (which we don't own) to run the game is different to that.

OpenRA is more of an engine than a game. The assets are just for the officially supported mods and as far as I'm aware it makes no use of the original software (unless you consider art a form of software)

It is a common naming convention for https://osgameclones.com/ and there are completely free mods like https://libregamewiki.org/OpenHV