A bare bone program with rendering and movement is not a game, it's a prototype, and this demonstrate nothing about modern game development. Of course a prototype with nothing but rendering and basic inputs coded in c++ is gonna be multi-platform by default. Hell, it is just code on a repo, you don't even need to build it and test it and deploy it for all platforms as it is up to the user. I don't think you understand the scope of making a fully-completed game. I had dozens of unfinished prototypes on my computer, some of which I made decades ago, some are multi-platform because of the language and tech. Still, this means nothing. It still cost money to support multiple platforms. Only exception nowadays is if your game happen to be compatible with Proton. But yeah, supporting Mac and a bunch of other platforms? It is not free my dude.