It’s only double counted in a situation where you’re actually counting both sides. This is a Canadian study published by a Canadian outlet about the impacts of Canadian policy.
They’re not trying to balance the books, so to speak, they’re evaluating transactions on a single account.
I’m curious what issue you see with that? It seems like the project is only accepting unrestricted donations, but is there something suspicious about shopify that makes it’s involvement concerning (I don’t know much about them)?