I don't think these terms are as different as you seem to be implying. When someone says "loosely coupled modules" they don't mean "printk" or similar functions. They mean such joys as power management and thread scheduling. These are somewhat modular in the kernel, to be sure. Are they so modular that you could TDD one or the other? My last understanding was not really.

Consider, you can have a device driver that runs fine "on its own" but crashes when run with another driver loaded. This is almost canonically the opposite of loosely coupled modules.