Aren't you making a general point here against public arguments that certain things lead to better outcomes than others? Do you reserve the same rancor for these little warning labels printed on packs of cigarettes about cancer risk? (Look at these non-smokers looking down on others...)

Public debate is, to a first approximation, nothing but arguing about which actions lead to better outcomes.

I could see an argument that the claim is that EA is insulting because it's akin to calling a chocolate brand "asbestos-free!"; you're implicitly suggesting that competitors contain asbestos. But note that this depends on whether the other brands do, in fact, contain asbestos, and EA is not shy about its claims that many charities do not effectively pursue good outcomes. It's hard to accuse EA of trying to sneak in this point when it's a central part of its rhetoric. And of course, such debate is an entirely ordinary part of discourse.

I draw a distinction between a particular charitable organization being effective, i.e. the amount spent on overhead vs going to cause they claim to support and a particular charitable cause itself being effective (mosquito nets vs. community theater).

For the former I agree, seek out charities that most effectively support what you want to support.

For the latter, I don't think it's worthwhile to deride people for supporting a cause just because you think there's some other cause that might technically result in more living humans. Raw population is not a worthwhile metric to optimize for. People should be free to support things they care about and believe make the world a better place as long as it's not harming anyone.

I still think there is a fundamental internal inconsistency here. I agree that people will, for instance, hold the moral belief that their community theater is worth more than five foreign lives. However, explicitly espousing this belief is politically unviable. The "common knowledge" moral view is that all humans are of equal moral worth. EA, as I understand it, is largely pursued by people who internalize this view; who recognize the mismatch and as a result, will prefer to sacrifice their community theater rather than admit inconsistency.