It is categorically different. OS threads are orders of magnitude more expensive, which makes them a nonstarter for most problems that are a good fit for lightweight conceptual concurrency.
As I said above, green threading has advantages over OS threading, but they behave exactly the same in terms of design patterns and potential bugs.
This is what I was getting at when I said "not that different": compared to the difference between event-loop concurrency and threaded concurrency, M:N green threading is basically just a subcategory of threading.
