FSFLA: Linux kernel is "open core"

That would be really bizarre, because by that logic, it was never part of the kernel to begin with (you could count it as part of the kernel if by that you mean "distributed with the kernel", but not as "part of the code which runs on the kernel").

Either you use the same logic on both situations (meaning the firmware was always part of the hardware, and so you cannot say it was ever really "part of Linux"), or you use it on neither (so it was "part of Linux" and stopped being so after being moved to the firmware tree).