It all comes down to intent. My DH and I frequently plan meals together and cook together. He'd have a good idea of what recipe I wanted to use, what ingredients I was adding, etc. In my case, if he added something, most likely he'd be helping me out and I wouldn't mind at all.
That said, sometimes I will have added a dash of this and a pinch of that and he'll come over and add a few shakes more without tasting it first
because when he makes that dish that's how much he uses. Sometimes that annoys me because I feel like "Well good for you, but so what? You could have at least tastes mine before deciding you could improve it, thankyouverymuch!"
It sounds like you and your DH plan meals and cook separately so I can appreciate that his gesture felt like "Let me fix this for you." and I wouldn't like that, either.
Our rule is "I cook, you clean." Maybe you two need a rule, too - "I cook, you keep your hands to your dang self."