I understand where you're coming from, because you're absolutely right from a conventional engine perspective. These engines are a different beast altogether, the rules force you to think from a different point of view. The limitations of normal engines are of little consequence when you consider the level of sophistication involved with these power units. They run on a completely different wavelength, here the key term is more heat = more power, everything else be damned, it just has to hold together for 4,000km then it can incinerate itself for all they care.
If you think about it, detonation is really the best way to get maximum power, detonation is so powerful that it can send a piston straight through the head, crack the block in half and take the body work with it.
Imagine if the engine could harness that power