I think "walled garden" in this sense is meant as an ecosystem of multiple apps that work together. IIUC Notion is just a single app, right?

Not really. There's an export option that dumps everything into a zip file for you. Multiple other apps have import from Notion ability, and it's also easy to read the exported files directly.

There's even a semiofficial API that works pretty well - you can read Notion notes in VSCode, for example.

Is a single plant a garden?