that armbar wasnt exactly perfectly executed. if you look back both of his legs were above tommy's chest. you have to have one leg over the face and another across the chest right below both arms. that was why tommy held in so long, because he wasnt being held down by ben's legs.
My friend who takes BJJ (actually its Arlovskifan) pointed out to me that the reason he lsot the armbar was because he shifted Tommy's arm the wrong direction. When you have it nearly locked it it feels like you should move it inwards to increase leverage but it actually loosens the hold. From what i was told he should have shifted it the other way because it would have completely locked the elbow and it would have been over. But im not BJJ expert so I cant really say.