In any case, I think the reason for the onion rings in this case is the aspherical elements rather than diffraction.
If the onion rings were present in every photo of every light source, and if every OOF blur circle produced identical onion ring patterns, I would agree. However the candle lights produce no onion ringing at all, and the various electric lights produce differing onion ring patterns. Given that, it seems more logical to me that the source of the diffraction is external to the lens and camera. In other words, it's caused by the light sources themselves...say the glass or plastic bulbs surrounding whatever is actually emitting the light.
My knowledge of bokeh structure is far inferior to my knowledge of optics. However, the link you posted itself suggests that the onion ring is due to aspherical elements in the lens rather than diffraction.
One general note: diffraction is not caused by the light source. It is caused by objects in the path of light. You are suggesting it is happening at the level of the impurities in the light bulb, I am suggesting it is at the level of the lens elements. Also, even the candle light is not a pure light source. The yellow part is actually incandescent carbon particles that don't burn fully. Lens elements behave differently with different sources of light (in other words, diffraction characteristics will vary according to the nature of light emitted), so while it may not diffract candle light, it might diffract electric light. I don't have the Physics knowhow to predict the exact mechanism.
I do agree, that the aspheric element can cause the onion ringing effect. However, and I am honestly trying to think logically here, would that not effectively REQUIRE that every light source exhibit the effect, and that every boke blur circle exhibit it in the same way? If the onion ringing, specifically in the case of Eldar's photos, was a consequence of the aspheric element, I truly do believe that the candles would have exhibited the effect as well. They would have to, since the effect is caused by the lens, and not a bulb or something else in proximity to (around or between) the light source itself.
Additionally, if you blow Eldar's sample images up, most of the effects on the blur circles themselves are inconsistent from circle to circle. If they were consistent, I would agree with you 100%, however they are not consistent. The lack of onion ringing in the candles and the inconcistencies with which each blur circle exhibits lead me to believe the problem is external to the lens.
That is not to say lenses cannot cause effects like this...they can. However I also believe that a $4000 lens would NOT be exhibiting onion ringing. It's a very noticable and ugly effect, very ugly effect, that I would not be spending $4000 on a lens that had that problem. I am a big fan of Canon glass, however I have great respect for Zeiss, and I cannot imagine them creating a lens like the Otus with such a nasty flaw.
To refer to the second paragraph- in short, light has been theorized both as a waveform and as a particle- not because one leads to the other, rather because it has characteristics of both (including diffraction). It is not purely a wave. So yes, diffraction is a property of light, which likens it to a wave.
Is there a difference in magnetic and electromagnetic waves? Yes, the former cannot proceed through vacuum.
Just to touch on a point. A magnetic wave is an electromagnetic wave. There is no electronic or electrostaic wave, nor is there a magnetic wave. There are electromagnetic waves. Magnetic fields
do indeed exist in space, which is a vacuum. NASA's space probes have been equipped with both a plethora of electromagnetic sensors as well as plasma wave sensors ever since the first ones were sent into space. We've measured the effects of electromagnetic fields in space, which includes the measurements of electric fields, magnetic fields, and plasma waves (electromagnetic effects propagating within free electrons and positively charged ions...plasmas...within interplanetary and interstellar space.)
Now, if you are referring to the propagation of a wave through magnetized mediums (say the waveform that forms in iron particles that conform to the electromagnetic field around a magnet), then that is a bit different. I guess that could be called a "magnetic wave."
But that is not relevant here. Light has characteristics of an electromagnetic wave.
I didn't say diffraction is limited to light waves. Light exhibits diffraction. Waves exhibit diffraction (quite independently). Hence light = waves. Is it that simple? No, because light has particulate properties, too.
You are right, light is not modeled after waves. Light is a wave. But then, it is also a particle. One doesn't imply the other, but they are clearly not mutually exclusive.
In any case I am not an expert in Physics, my field is Biology. My knowledge in Physics is quite limited, and I type rather slowly, so I shall stop here. But the information above is quite accurate as you can check out.
I think we pretty much agree on everything else.