The question is not whether charitable giving can accomplish good things: of course it can. I don't think anyone would say that, for example, preventing malaria, treating parasite infections, and providing people with clean drinking water aren't good things.
The question is, why do we pretty much all agree that those obviously good things are obviously good things to do? Do we agree because of some abstract "rationalist" calculation? No, we agree because of obvious human ethical intuitions: wouldn't you want to be free from malaria, have your parasite infection treated, and have clean drinking water? The only "rationalist" calculation that comes in here is figuring out how to give such things to the most people for a given amount of money, and that also doesn't require any huge abstract calculation: it just requires looking at obvious metrics of efficiency, like minimizing administrative overhead and minimizing the impact of the corrupt governments in the countries where these charitable efforts take place.
In other words, we don't need Derek Parfit or any other ethical philosopher to tell us how to be "effective" with charity when the things we are doing are things that pretty much everyone agrees are obviously good things. It's nice if the ethical philosophers agree that those are obviously good things--but it's not clear that Derek Parfit, for example, would agree, since he can't even agree that it's a good thing to not treat people close to you callously.
Also, even if they agree on the obviously good things, the ethical philosophers go way beyond that. They argue for ethical claims that are not at all obvious--that most people would say are obviously wrong--and they do it based on abstract "rationalist" calculations even when those calculations fly in the face of the very ethical intuitions that lead most of us to agree that things like preventing malaria, treating parasite infections, and providing people with clean drinking water are obviously good things to do. But if you discard those ethical intuitions--and certainly someone like Parfit who is willing to treat those close to him callously, and do it intentionally, based on his ethical philosophy, is doing that--what do you have left? What basis is there for making any kind of ethical claim at all? At that point, whatever concept of "good" the philosopher is arguing for is just made up out of whole cloth, based on their personal ideology.