I was surprised to see mention of MIRI and Existential Risk. That means that they did a little research. Without that, I'd be >99% sure it was a scam.

I wonder if this hints at their methodology. Assuming it is a scam, I'd guess they find small but successful charities, then find small tight-knit communities organized around them and target those communities. Broad, catch-all nets may catch a few gullible people, but if enough people have caught on then perhaps a more targeted approach is actually more lucrative?

Really, it's a shame to see this happen even if no one here fell for it, because now we're all a little less likely to be receptive to weird requests/offers. I suspect it's useful for EAs to be able to make random requests of specific people. For example, I can imagine needing a couple hours or days of consulting work from a domain expert. In that situation, I'd be tempted to PM someone knowledgeable in that area, and offer to pay them for some consulting work on the side.

I can actually think of 2 instances where this community has done things like this out in the open (not PM), so it wouldn't surprise me if there are occasional private transactions. (I'd link to examples, but I'd rather not help a potential scammer improve on their methods.) Perhaps a solution would be to route anything that looks suspicious through Bitcoin, so that the transaction can't be cancelled? I wouldn't want to add trivial inconveniences to non-suspicious things, though.