Without knowing him, it’s hard to know if he was being a knob or not.
Sounds to me like he was just trying to be helpful & your reaction does sound a bit OTT. But, it all depends on his attitude etc.
FWIW, I fairly regularly see cars with bonnets open on the latch – perhaps two per month. If I am in a position to do so, I will tell the driver that their bonnet is open.
The response is either 50% grateful that I’ve pointed it out or 50% ‘no it isn’t (you knobber)…..oh, erm….’.
From the sounds of your reaction to hearing what this bloke had done, I suspect you’d react in the second manner….?