> Epic has no "walled garden".
Epic is explicitly a walled garden regarding the games it has paid to be exclusive within that walled gardren (Permanently or temporarily).
> The PC is an open platform. An iPhone is not.
You are conflating hardware / software here. PC hardware is an open platform. The software is running on it is not necessarily. You could be running Linux or Windows for example. (Of which only Windows has an official Epic Games Launcher client).
> If I don't like the Epic store, I can install software from other sources just fine.
We are talking about the context of games so "software" means a particular game of which there is only one. Not "Any random note-taking application". Therefore, in the context of games, no you cannot get that software (game) legally from any other source if Epic paid for it to be exclusive.