You don't have to predict a misfire. The Green Missile is in fact a chargeable attack, and when charged it does have kill power. You can force a foe offstage, read their recovery well in advance, and partially charge a Green Missile. You can also use a Green Missile as if you were going to end a combo by hoping for a misfire, but instead bait the airdodge by charging the move while falling right next to the opponent, releasing as their air dodge ends.
And while Luigi's air speed is legitimately terrible, it seems to me as though his air traction is among the best out there. He's able to quickly change direction or adjust his speed in the air.
That's partly luck, and most likely partly Nairo/HBox fighting an unfamiliar opponent and not yet knowing their patterns. Plus, comparatively new players occasionally make technically sub-optimal plays that can still take an advanced player by surprise.