Maybe the coaches instructed him to go out and empty the tank all guns blazing for 50 minutes. And maybe they trusted the subs to finish the job. You can ask yourself if he would have been able to continue at this pace for the last 30 minutes.
Everything 40 years ago was not like it is now. Hell, there was no lifting in the lineout and flankers were allowed to leave the scrum before the ball was out. Besides, as mentioned before: this was not a clash of 2 players going for the ball but a tackle on 1 player jumping for the ball. »