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What impact does being marked as "originally installed by a browser or file manager app" have? And how would that be different for Epic Games Launcher/Store?

The article mentions that even standalone apps can update themselves without user permission if they follow the requirements, Epic Games Launcher/Store is also a standalone app that needs to update itself at some point.

It doesn't change the initial installation dance with allowing "unknown sources" with all it's "scary" language and hoops a user has to jump through it seems; maybe that's what you meant? At least for Epic this will still be a problem as they see this as a clear disadvantage: https://www.theverge.com/2020/4/21/21229943/epic-games-fortn...