Yeah, weight class. It's not really an "unholdable" guard break because the guard break is used with a power blow. The power blow is unholdable and if they attempt to hold is gonna behave as a power blow. The guard break perk kicks if they respect the power blow.
I found another with 1P+K and it doesn't work on heavyweights.
Whenever you launch them with 8K, throw out 1P+K at full charge, it's unholdable and she gets +5 advantage. If they tech roll backwards, they can hold 1P+K.