The first usage I remember of walled garden in an online sense was about AOL and their refusal to use common internet protocols - you literally could not email from outside AOL and if I remember not view websites outside AOL. There was a wall around their garden. IIRC it became a common description in tech columns of papers.
I still see the walled garden analogy to be more useful
If people are confusing their definition of a metaphor and mine (I don't think Orwell mentioned that but maybe it's low down the list of stylistic errors) then that can cause problems. But I struggle to see how Facebook / twitter "protects" creators from the harsh winds of the outside.
This is where the "dying metaphor" thing comes into play: AOL was a walled garden in the sense that I outlined - like so many other early online services (and BBSs), it had walls to protect its members. That was the original sense of the metaphor.
But... That sense is dying; it is a poor metaphor because few people actually build real-world walled gardens[0][1] for that purpose; the metaphor has no currency, and thus the meaning shifts. Now it is just as frequently used to describe any sort of system which restricts the flow of people OR of information, in or out. So while Instagram might be considered a "walled garden" in the original sense (no outbound links on posts), it may ALSO be considered a "walled garden" in the sense that it restricts inbound access for non-members, or even in the sense that it forces certain onerous licensing terms on contributors. In this manner, the metaphor becomes problematic, as what one intends to convey is not necessarily the meaning which is understood by others. If/when the metaphor dies entirely, becoming an idiomatic way of saying "not completely open", this problem disappears - no one will attempt to relate people to plants, or content to flowers.
With this in mind, I suspect your original intent was focused on the "garden" aspect: that the value MO provides comes from imposing a structure and certain expectations which facilitate productive interactions like this, with the "sweet spot" being that it remains open enough for the rest of us to benefit from the outcome of those interactions.
I still see the walled garden analogy to be more useful
If people are confusing their definition of a metaphor and mine (I don't think Orwell mentioned that but maybe it's low down the list of stylistic errors) then that can cause problems. But I struggle to see how Facebook / twitter "protects" creators from the harsh winds of the outside.