YOu don't need a zero deck for a quench engine.

Imagine if you will, a closed chamber head and a flat top piston. If the clearance between them is small enough at TDC you will have quench. Very generally speaking .035-.055" is where you want to be. The tighter you go the greater the possibility you will smack parts together, the looser you go the less quench effect. It take good machining to set this up.

No, why quench? The theory is that a tight quench squishes the air fuel mixture as the piston rises mixing it more thoroughly and lessening the possibility of a lean area in the mix detonating. Another theory is that the closeness of the parts help draw excessive heat out.

Another variant is a double quench setup, with what amounts to a trough shaped piston and combustion chamber, this forces the air/fuel mix into a more compact shape that optimizes combustion speed and rate, lessening the chance of detonation. A perfect sphere is the most compact shape for a given volume, so the closer you can get the air/fuel mix to a sphere the better.

Not sure if any of those theories really explains how it works, but it has been well documented that it can help. Mind you all of the above is a gross generalization.