Can you really win by just doing that? I tried it and it didn't work for me, your mirror might also collide with yellow particles or worse, even a thread that fails right away. It looks like it may be better to use the threads. If it's possible to win with just collecting the green ones it's good you were able to do it.
I didn't look at this comments section, somehow I figured out the thing with the thread on my own, but it took me a while to figure out, and a while to get good enough at it to eventualy win a round, I wonder if it should be explained or shown somehow just a little bit in the tutorial a bit more than it is now.