If on average aid has not contributed to economic growth, and the best foreign aid charities positively contribute a lot to economic growth, then as many other foreign aid charities negatively contribute a lot to economic growth

My impression is that the situation is closer to a very large majority having small negative impact and a very small minority having a large positive impact.

people cannot tell them apart (if they could, they would definitely shift their contributions).

The reason that people cannot tell them apart is that they're putting essentially no effort into doing so. According to the recent Money for Good study only $4.1 billion of the $300 billion donated mentioned in the above was donated by donors who do research comparing multiple charities when deciding where to give. It's plausible that donors who make an active effort to maximize the positive effects and minimize the negative effects of their donations can do far better than the average donor.

what terms of the bet are you proposing as I'd take it if it wasn't for difficulty of measurement.

I'm not literally proposing a bet; I'm just saying that while it could be that donating to charities like Deworm the World and VillageReach doesn't have a positive impact on economic growth, I judge the expected value to be moderately positive and I don't see any reason to think otherwise.This is in line with MassDriver's comment

honestly I find it hard to even articulate a hypothesis on which, e.g., de-worming initiatives don't foster economic growth. I wouldn't be starting many local businesses if my brain couldn't get calories out of my gruel because they went to a tapeworm first.

There are plausible explanations for why the net effect of aid has been trivial that don't preclude the hypothesis that the interventions under discussion are effective.