How is that luck? He obviously heard him walking above and had the patience and awareness to know when to pop out and shoot. No luck at all. Hes just being a Pro.
Flashbangs make a loud whining sound so he wouldn't have heard him. You might have got about 3 seconds of just making out footsteps so he pops up, sees him and no-scopes. Half luck half skill. Just in the right place at the right time i guess.
Thats not hard it was very close range and he heard hi walk just jump up and click your left mouse button. My bro even could do tht LOLZ