Hi Peter,

The scene complexity might not be a big issue in itself with the advancement of graphics hardware. But, what the author wants to say is that, an algorithm shouldnt ideally depend on scene complexity which can be arbitrarily complex. So, no matter how fast graphics hardware is used, there is always a finite probability that the scene wont be rendered as fast as we want. One of the key words in definition of realtime systems is " Predictability". Hence,
it is essential that the timing constraints of the system are guaranteed to be met. If a rendering algorithm is scene dependent, no guarantee can be made about real time performace. For the sake of completeness, here's a classical canonical definition of real time systems:

"A real-time system is one in which the correctness of the computations not only depends upon the logical correctness of the computation but also upon the time at which the result is produced. If the timing constraints of the system are not met, system failure is said to have occurred."

Others have added:

"Hence, it is essential that the timing constraints of the system are guaranteed to be met. Guaranteeing timing behavior requires that the system be predictable. It is also desirable that the system attain a high degree of utilization while satisfying the timing constraints of the system."