The article says: “In its simplest form, you use it as in the following example:
<permission type="camera" />
It's declarative in that it's telling the browser to render a complete UI for showing and modifying the state of the permission. Just because there's also an imperative API that already exists doesn't mean it's not declarative. It's certainly not imperative, there's nothing imperative about it.
Moreover, it allows the user to show the browser controls for re-requesting the permission if it was previously denied, which isn't possible with the imperative API (because an imperative API can implicitly be abused).
Except it is declarative? The element renders a button and clicking the button (without any JavaScript) shows a browser UI for managing the permission. How is that imperative?