The iPod wasn’t a success because of the walled garden. They didn’t have one (once they released on Windows).
If MS had released a player that was good enough they could have stolen the market, yes. But that’s the point. No one did because everyone released something akin to a flash-based CD player. They didn’t rethink the interface much. The interface is why the iPod grabbed the hold it did.
Yeah it became cool over time.
But I contend an iPod with the interface of a Creative Nomad Jukebox wouldn’t have succeeded the way it did.
My reading of the upthread comments is that people are saying a walled garden isn't necessary, and using the iPod as an example of Apple's success before it had a walled garden. No one is saying that they had a walled garden back then. They did have a built-in audience (Mac users), but that is different from a walled garden. On iOS, they can prevent app devs from accessing their hardware in the same way they can, effectively neutering their efficiency.
Apple can afford to be a much later mover, even against very well-resourced competitors, now that they have a huge walled garden. If they didn't have the walled garden, it would be very difficult for them to catch up.