My Caedmon has 714 base attack.
The troop give 20% attack bonus.
Caedmon attack without buff = 714 × 120% = 856
Boldtusk buff +48% attack = 48% × 856 = 410… It’s 406 in reality due to rounding.
With 856 attack stat, Caedmon tile hit Kappa around 50 damage while the special hit for around 600 damage.
With 856 + 406 = 1262 attack stat, Caedmon tile hit Kappa around 85 damage while the special hit around 700 damage.
It seems you are correct when you said that the 48% were added to %damage. But that would make it as a bug as it is against the attack stat displayed.