EA risks falling into a "meta trap". But we can avoid it. (EA is effective altruism. It is favored by people who believe they are into reason and logic, similar to the Less Wrong community. The standard type is an atheist who rejects superstition, loves science, talks about thinking fallacies and biases, and reads non-fiction books.)
While donating to AMF all our lives is great, if we can spend our effort to get two people to donate to AMF instead of us, we’ve doubled our impact.
The author means: instead of person A donating $100 to charity, he can spend $100 on marketing/outreach/persuasion to get persons B and C to each donate $100 to the charity. He claims that, if that works out, then it means person A has doubled his impact.
Is that good? I think it means person A doubled his impact at triple the total cost. Now $300 was spent instead of $100.
This is the same issue as the broken window fallacy, also known as the fallacy of the seen and unseen. (The seen is the window repair guy getting paid and then spending that money at a bakery and then the baker buying shoes and so on. The unseen is that the window owner would have spent the money on something else if he didn't have to buy a window repair, e.g. he would have bought a suit and then the tailor would have bought bread and then the baker would have bought shoes and so on. So breaking the window did not stimulate the economy by creating demand for window repairs and thus make people better off. Breaking windows is bad.) The seen here is persons B and C donating $100 each to the charity. The unseen is what they would have used that money for otherwise. That $200 would have been spent elsewhere, and might have provided more value than an additional $100 for the charity.
(If you don't recognize my explanation of the broken window fallacy, and want to learn more, read Economics in One Lesson. I'm just repeating economists like Bastiat and Hazlitt, not saying anything new. The book description at the link states 'this is the book that made the idea of the "broken window fallacy" so famous'. It's a great introductory book which doesn't require doing math.)
In the author's math, the $200 spent elsewhere has zero impact. It's worthless. That's not a considered opinion, it's because he forgot to count it for anything, he didn't think about it, just like the broken window fallacy forgets to consider what the window repair money could have been spent on instead.
Suppose the $200 was spent quite badly, then maybe it would have $50 of impact (25% effectiveness on a scale where the charity is 100% effective). That's generous and lets his meta strategy come out ahead, but not by double. Let's do the math on how much the person A actually helped anything. If he donates $100, and the other people spend their money badly, the total impact is $150. If person A does marketing and gets B and C to donate, the total impact is $200. The increase in impact in this generous scenario is 33% not 100%. (100% impact means double, that's the claimed impact.)
If B and C would have spent their money at 50% effectiveness, then everything comes out equal. If they would have spent their money at 75% effectiveness, then person A hasn't double his impact, he's made the world worse.
Also, charities can handle their own marketing. If you donate $100, the charity itself can then use that for marketing and bring in $200 of donations. If they don't think more marketing is the best use of that $100, there is a reason.
Some charities seem happy to spend $100 asking for donations in order to bring in $101 of additional donations. This makes the world a worse place! A lot more wealth gets spent on mailing letters and other things that don't help people.
The author thinks spending $100 to bring in $200 of donations is a $100 win. By the same logic, spending $100 to bring in $101 of donations is a $1 win. He'd see it as a positive thing because he forgets that those $101 of additional donations would have been spent on something else that would have been a larger win than the $1 benefit he sees.
Conclusion: The EA community is grossly incompetent. It's not just this one writer (who participates in EA discussions a ton), it's the whole community, or else he would have been corrected (the post was high effort and got significant attention, and there are a bunch of very positive comments). They are literally doing broken-window-fallacy level thinking while believing they are cleverly improving charity, and the whole big community of "smart" people do not see and correct the error.