I think the reason Buccellati immediately assumed the Boss was the one who attacked Trish instead of the last pursuer, is because of the circumstances behind the attack.
The pursuers are that - they're tracking and chasing the gang without knowing where they take Trish. This attacker ambushed them in a place the gang didn't even know they'd go to. The Boss was the one who called them there, instructed them to use the elevator, and said that he'll get his daughter on the top floor, which is what happened.