Does winning at a −7% disadvantage prove my build is good in Dragonfire?

No. It's a stubborn assumption. Yes, wins happen at a −7% disadvantage (e.g. Shieldbearers 14,573 vs Cavalry 19,500 at Greywater Watch). But look closely and those wins are mostly explained by attrition: earlier armies had already weakened the defender.

The defender never heals, so a pre-softened target falls even to a mediocre build and an unfavorable matchup. It's not that the −7% doesn't count — it's that the earlier wear did the work.

The correct nuance: the ±7% is a real but small factor. Without the prior wear, the outcome becomes uncertain again and depends on the real variables (power gap, auras, Initiative, number of rounds).

So never conclude "disadvantage doesn't matter, my build is great" from a fight against an already-softened target. To judge a build, look at a fight against a full defender.