z in the US is pronounced zee, in the UK zed, so certainly not an example of onomatopoeia for Brits, or a particularly good example for Americans. It's certainly a stretch to attribute it this way.
I don't know about in American, but in the UK it's not that "zed" is onomatopoeic, it's "zzz" which is just the sound a z makes in a word, not "zedzedzed" or "zeezeezee". And that's why it comes from snoring.
