A somewhat more detailed description of the botulin organism is at:

Clostridiumbotulism.pdf from the WHO.

Botulin toxins would probably be fairly unlikely in stored milled cereal products, since the bacterium requires anaerobic conditions for growth. It does grow in soil and is sometimes found - as pointed out by the Mayo brief - in honey and corn syrup. Since it's actually fairly common in the environment, it could grow enough to produce harmful concentrations of the toxin in flour or other cereal products stored in sealed airtight containers, or in storage where bulk quantities are sufficient to contain air-deprived areas. Simple oxidation of the "flour" could provide sufficient reduction in air exposure for growth, if there's no "stirring" to re-aerate regularly.

Because of the need for relatively anaerobic conditions, botulin producing bacteria frequently thrive, as in the soil, in the presence of other decay processes that "suck up the air."

Since the bacteria (there appear to be several that produce the toxin) do not generally grow in acid-rich materials, the baking pre-mixes could have some enhanced likelihood of supporting growth. Most such mixes contain fairly liberal amounts of "baking powder" of one kind or another, most of which have a fair amount of simple Sodium Bicarbonate, which IIRC buffers at a slightly alkaline pH.

The WHO .pdf indicates a requirement for 250F (121C) for 30 minutes for safely killing the botulin producing bacteria for canning. That temperature of course requires a pressure cooker - at least, and while that may kill the bacteria it's unclear whether even that will "detoxify" any toxins they've already produced in food that's "slightly spoiled."

John