True, but writing a linear sort with bounded number of distinct elements is not really a challenge.

E.g.

Intput, T: array of n elements, m: number of distinct elements.

    for i in 1..m
       for j in 1..n 
           if( T[j] == i ) print T[j]; 

This is O(n * m) which is O(n) since m is a constant.