The law requires them to be both subjectively and objectively reasonable. Not perfect, no, but objectively reasonable. It is not enough to be afraid because of what someone might conceivably do, but hasn't actually done.
The texter was engaged in a physical attack. Should the person being attacked take notes or ask, how far are you going to take this attack?
The person who happened to be 71 has the right to defender himself. The attacker guessed wrong and was killed. Too bad.