Does this mean that the app could be forked to support multiple accounts (which is currently not possible for the free tier users with the official app)?
ProtonMail can put this app on the store, because they own it and can relicense the App Store build at-will. But no one else owns ProtonMail's code, so we can't take it, give it an App Store-compatible license, and publish it.
Also, it only seems like a simple task. You still have to manage notifications and UI for multiple accounts. Not to mention that I’d probably not do that free of charge.