As I understand it, the designer used freertos. At that point, he can just give a readme with hand-wavey "get sdk, install freertos, find firmware files here...

As I understand it, the designer used freertos. At that point, he can just give a readme with hand-wavey "get sdk, install freertos, find firmware files here... and here's all my code which I'm free to license as I wish" and no license infringement necessary. Not sure why a backdoor is needed, but then I've never built a project like this.