What zeno said, mystery cooldown increase isn't a debuff. To dumb down what a debuff is, consider the physical change of the character that you can see. You can see the status effect of poison, ignition, etc. You can't see mystery cooldown increase or stat debuffs (intiative, defense, resistance, crit) on the character so not a debuff.

The situation you described would mean kimimaro is faster than summer kurenari, which means not only does her mystery gets interrupted, her mystery cooldown increases. So summer kurenai doesn't do anything from that round.