This ethanol is produced via fermentation, though. Just not fermentation of plant matter. The OP uses the word "fermentation" several times.

We don't normally think of the containers where wine or sour mash are fermented as bioreactors, but they are.

Right, but the source product is sugar a C6 compound, and the fermentation pathwasy removes carbon atoms. This process is building up from C1 so methanol is a possible side product. Or at least I bet that is the concern

Ethanol produced by fermentation also contains impurities and methanol. The FDA/ATF say in the link that the majority of vinegar and alcohol used in food cone from synthetic sources.

I haven't read the link yet, but did not expect synthetic alcohol to be used for food and beverages.

I was surprised at how widespread it is.

Ethanol produced via fermentation tends to have methanol as well though. IIRC wine has a fair bit in it.

I am not an expert in traditional fermentation but I believe presence of contaminants depends on the crop, the strain of yeast present during the process, and conditions. So basically it can vary a lot.

Methanol and other byproducts do vary based on strains, conditions, etc. Generally, methanol is not an issue in things like wine because it's not produced in large enough quantities, even when conditions are poor. A treatment for methanol poisoning is actually ethanol (and likely some NAC). Only when distilling is methanol concentrated enough to cause problems. When done correctly, the distillation process separates the methanol from the other distillates at specific temperatures. Usually the heads and tails of a distillation run are thrown away. I believe it's the head that has the lions share of methanol.