Remix OS is based on Android and Android includes, among other things, the Linux Kernel, which is GPL, which is Copyleft, which would mean that the Remix OS creators would need to provide the source code to users, wouldn't it?
Example: If I wrote an alternative GUI system to replace X and the open window managers, I wouldn't necessarily be forced to open-source that system.
Real-world example: My employer uses Linux in some of its products, integrated with our proprietary software in license-compliant ways (and we've got a legal office that verifies that we're following the licenses for all the software that we use, to avoid legal trouble).