Hi Bob,
Interesting that article was written in 2003. Much has changed since then even though 2003 doesn't seem that long ago. Yes, a zener diode was used to suppress peak voltage in the primary it seems. Could have been a couple of reasons I can think of for that. The author indicated he would follow up with more info but evidently didn't.
As for resonance, don't you think they'd be talking about the resonance of the RLC circuit? When the points are closed, the primary is part of an RL circuit, yes? Then when the points open, an RLC circuit is created. From there I think it has to do with running the secondary at the proper resonance to make the most of the coil's energy...and the only see-saw comparison I can think of quickly here is tuning the idle ratio of fuel/air in a carburetor. Because the inductance is fixed (the air intake in the carburetor) and the capacitance can be changed (the idle richness screw), the capacitor becomes the regulator of that resonance. Maybe the use of the word "tuned" means nothing more than choosing the right value to get the most "oomph" out of the coil.
