Sure, there are advantages to both approaches, if done well. But I bet that diving into Linux first would make it really hard for beginners to get a deep grasp

Sure, there are advantages to both approaches, if done well. But I bet that diving into Linux first would make it really hard for beginners to get a deep grasp of core concepts; may be fine for more advanced learners. e.g., the scheduler is probably 10,000,000,000 lines of complexity when the core scheduling algorithm probably fits on one page of code.