Understanding the P2||CMax scheduling problem
Taking apart the notation, here is what we have:


Two identical machines. 
$n$ jobs with equal duration $p$.
No release dates or deadlines, i.e. $r_i = 0$ and $d_i = \infty$ for all $i$.
No extra features.
Objective function is maximum completion time.


It is true that all schedules without idle time have the same completion time in this setting. However, it still takes linear time to actually write down a schedule. 

I guess this an easy exercise that is supposed to get you accustomed to notation and working in the model, and will maybe be extended to more complicated $\beta$.

Also, algorithms for this problem may be used as a subroutine, for instance for computing lower bounds in branch-and-bound algorithms for $P_2 \mid \beta \mid C_\max$ problems with non-trivial $\beta$.

Note: The number of processors can make the (conjectured) difference between polynomial and NP-complete complexity.