Most of the rest of the game is already probably visible in plaintext. And the big thing is, they're not releasing the copyright to their intellectual property around Command & Conquer: So to legally use this, without providing your own assets, you have to own a copy of the game.
But presumably, if you own the game, you now have everything you need to modify it however you like.
That is my point.. reusing or remixing the assets, like their "mammoth tank but with another turret" example, is just barely fair use, provided it is a free2play mod on their engine. All this talk about "open source" sounds nice, but 95% of the game is still "proprietary" and most of the code is still closed and the whole thing is still mostly about selling the enhanced edition. Which is fine. The news is just very misleading until you read the fine print.
But presumably, if you own the game, you now have everything you need to modify it however you like.