You are incorrect. Generating trajectories is easy. Many well known techniques exist that do pretty well, and yes this is done in computer games all the time (as well as in many other fields).

Quickly generating time-optimal trajectories for under-actuated mechanical systems with actuator constraints is interesting, and as a researcher in this field I can assure you that the technique in this paper is novel and is interesting—if it were not it wouldn't have been published in the journal Science...