I think what you're describing is vertical integration rather than the walled garden specifically. The walled garden is the App Store restrictions, iMessage lock-in, that kind of thing. What made the Neo possible is that Apple controls the silicon, the OS, the firmware, and the industrial design as a single unit. They could put a phone chip in a laptop form factor and have it feel coherent because there's no seam between the hardware and software teams.

The distinction matters because it changes what the lesson is for the rest of the industry. You don't need a walled garden to compete here. You need to own enough of the stack that you can make aggressive tradeoffs (like shipping 8GB and an A18 Pro) without everything falling apart at the integration boundaries. Microsoft can't do that because they don't make the hardware. Dell and Lenovo can't do that because they don't make the OS. Qualcomm can't do that because they don't control the software ecosystem.

The one company that could theoretically pull this off is Google with ChromeOS on their own Tensor chips, and the fact that they haven't is probably the more interesting question than why Asus is shocked.