Perhaps our distant ancestors also tended to eat a small bite first, then proceed with other foods, before consuming the remainder? For that matter the drug wouldn't be absorbed all at once. If it is toxic better to eject whatever hasn't been digested than continue absorbing it. There's a bonus psychological side-effect: vomiting is not pleasant and the individual (if they survive) will be more likely to remember what they ate and how sick it made them to avoid it in the future.
I never want to do it, but it nearly always makes me feel much better. You can even get a sort of sense of calm or euphoria from vomiting. I can't say that it's pleasant, but if it were less gross and smelly it wouldn't be so bad... although the muscle contractions would still be hell on my under-trained abs.
I'm familiar with the sensation; it's enough to make you feel better, possibly even sober (if it was a drinking related episode), but the effect is unfortunately very temporary.
Oddly, I gather rats cannot barf, which is why coumarin works as rat killer.
I think your recollection is off by a little. Coumarin is an anticoagulant and damages rats' livers: https://en.wikipedia.org/wiki/Coumarin
You might be thinking of corn gluten meal, which kills rats by preventing them from absorbing water? It's sold as RatX: https://www.ecoclearproducts.com/pages/ratx-faqs