It's a walled garden in the way that matter most, which is that if the things inside the wall get back, you can't take your account/social graph and go outside the garden.
It's generally difficult to make people move away from well-established platforms. Would Matrix be a walled garden if it was the leading communications platform?
Exporting all your account data is very easy with Discord, and you can even export entire servers with bots. Doesn't seem like they make it particularly difficult to move away from their platform.
