Leaving buttermilk out overnight, recipe for food poisoning?
Let's assume that we're talking about cultured buttermilk (the modern, sour kind that smells like yogurt), rather than the old-fashioned by-product of making butter, sweet cream buttermilk.

Cultured buttermilk is pretty acidic, with a pH of about 4.5. Acidity is one of the conditions that control the growth of foodborne pathogens and 4.5 is outside of the optimal growing range for most of the bad-guys. 

If the buttermilk is fresh, it should have a living colony of good-guy, lactic acid bacteria. This should mean that as it warms up, your existing good-guy bacteria will multiply and lower the pH a bit more. Lactic acid bacteria (the guys that make cheese, yogurt, buttermilk, and sour kraut) help preserve things by making the food a hospitable environment for themselves and an inhospitable environment for bad-guy, pathogenic bacteria.

Since there are existing good bacteria, the pH is low, and neither the dry corn nor buttermilk is likely to bring in contamination from bad-guy bacteria, this instruction seems reasonable. 

In fact, maybe the author wants the buttermilk to get a little more acidic (sour) overnight by allowing the lactic acid bacteria to develop a bit more. 

That said, the buttermilk package says Keep Refrigerated, and it would be possible for contamination with a bad bacteria to happen and for pathogens to multiply overnight. So there is some risk.

You could soak it in the refrigerator if you wanted to play it safe. 



Sources: reading about making cheese, making cheese, watching buttermilk get thicker and more sour in the fridge over time.