Packaging a mod with a engine so it works separatelly is a old tradition. Selling a game that way, that originally was a mod, is nothing new.
Not much different than Counter-Strike and Team-Fortress 2*.
At the same times, "It looks like a mod" used to be a insult. Journalist and gamers would use it to describe a game soo poorly done, that looks like a hackjob done in a weeked by two teenagers. People don't use it that way anymore, but the idea persist that a mod is a hack. A mod is not something you expect to work always and to be perfectly well tested.
Theres nothing wrong with doing that. The question is.. is what they releasing fun?