I think his point is that if Student A started with 50% knowledge and Student B started with 80% knowledge and by the end they both have 95% knowledge, Student

I think his point is that if Student A started with 50% knowledge and Student B started with 80% knowledge and by the end they both have 95% knowledge, Student A's grade for the course is penalized for the slow start even AFTER reaching mastery by the end. If Student A ends up only at 80% then so be it, but if they end up matched then they should end up with a similar grade.