It is a classic platform bootstrapping model. It worked with new book publishers in the 1800s too. It worked for new video game consoles, etc ...
The content creator doesn't fully trust the new publisher/platform, so they need upfront monetary assurance beyond a normal advance/royalty type deal, in case the platform fails. Likewise the platform values exclusives from big names that will drive demand for the platform. So the platform gives up potential upside to secure the exclusivity rights and the content creators get a premium.
I've seen a fair bit of discussion about this, and so far, every author who I've seen talk about this has said they took the deal and came out behind, or they refused the deal but would have come out behind if they'd taken it, ie, subscription revenue ended up exceeding the offered advance.
I imagine there must be some authors out there who didn't mmet Substack's expectations, but they're apparently few and far between.
So in those cases, Substack provided financing and made money on it in the end. I don't know if that's true on average, though.