What is the actual probability of food poisoning with food left unrefrigerated overnight?
I'm not sure how answerable this is, as kitchens vary so much. 

A clean, cool kitchen will be very different to a steamy hot, dirty one, in terms of both likelihood of contamination and growth of pathogens after the food is contaminated. Some pathogens are also almost ubiquitous. Homes may be cleaner than they used to be, but in temperate parts of the world they're also warmer. 

It doesn't of course help that the guidance is almost all written for cases where for is being provided for the unsuspecting public (including individuals at greater risk) rather than in the home. 

And then it depends on the food as well. Some support the growth of particularly bad species particularly well (e.g. cooked rice), others tend to spoil in detectable ways before they're colonised by nasties (many dairy products and actually bread).

From my experience I can say that the risk from food left out is really rather small compared to other sources of food poisoning - but a sample size of one is no use. Even if it's not a threat to your overall health, food poisoning is very unpleasant so it's worth a little effort to avoid it by cooling hot food quite quickly and chilling it. I sometimes forget stuff that starts too hot to put in the fridge, but don't waste it, and that's how I know the probability has to be low. That's not a recommendation, far from it.