Echoing student ranks
Since you use it in a subquery, I don't see the problem. If you used both in the same query, that would be redundant. But the subquery needs to use it to obtain the required data.

Did the person give a reason as to why he/she thinks this, or was it simply a general statement?

The only thing I can think of is that DISTINCT is rather slow compared to GROUP BY: stackoverflow.com/questions/1887953/