Calamari or squid is of course famous for being difficult to cook, because it gets tough or rubbery.
As Harold McGee explains in On Food and Cooking, octopus and squid meat are very rich in collagen:
They are chewy when lightly cooked, tough when cooked to the
denaturing temperatures of their collagen, around 120 - 130 F / 50 -
55 C, and become tender with long, slow cooking.
The trick then is to either:
Cook them minimally, so they do not begin to toughen. This Serious Eats recipe for fried calamari recommends no more than one minute.
My own interpretation of this is that it will keep the temperature of the squid below the 120 F threshold, at the risk of being below the pathogen kill point, so it should be done only with squid from a trustworthy purveyor.
Cook them for a long time, slowly, as in a braise, so they move through the tough phase to tender again. This Food Network recipe for stuffed squid by Ann Burrell is an example. The total squid cooking time is 20 to 25 minutes.
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.
Best Answer
You can get food poisoning from every kind of food.
The thing with Calamari is that if you cook them too long they become rubbery.
Nailing the proper cooking time is something that can be learned by experience.
See this thread: How do you cook calamari / squid and avoid making it tough?