I'm not sure which is correct. Not that familiar with utilitarianist nuts and bolts.

As with so many things, if there is more than one way to interpret something there is generally not too much to be gained by interpreting so that there is an error when there is a way to interpret it that makes sense. Clearly if a new charity sets up that takes twice the cost to provide the same benefit, and people switch donations from the cheaper charity to the more expensive one, utility produced has been decreased compared to the counterfactual where the new more expensive charity was not set up.

So whatever terminology you prefer, 1) opportunity cost is a real thing and arguably is the only good way to compare money to food quantitatively, and 2) whatever the terminology, the point of the original article is a decrease in utility from adding a charity, which is a sensible idea and well within the bounds of reasonable interpretation of the title under question.