Well, according to the Q and A
Simply put, Drake harming himself through Malga's physical buffing spell caused the Testament Arma to see actions that would have the wielder being harmed as "not against England's justice"
Gin used this against Drake. Her first attack would fail from the Testament Arma, and at the point of failure, she chains her next attack.
The Testament Arma cannot resolve the contradiction of its first judgement and the outcome being something that sullies England's justice and so fails to negate Gin's attack.
It's actually Malga's victory when you think about it....