Thinking a little more about the "sub in hand", I could see an argument being made that the substitution not be used in case it was needed to replace an injured player (kind of like not using your backup catcher as a pinch hitter) -- there's no way Arena would want to be caught 10-on-8, even for a few minutes while a player got treatment.
Is a man who is dead on his feet much better than no man at all? (Personally can't say in this case - I'm playing devil's advocate).
If anything I'd have used a sub just to waste some time, which it does even if it shouldn't in theory.