Elsewhere, @spectranaut noted:

because "accessibility tree" alone is defined in ARIA and it basically means the platform API's tree.

I think I'm still missing something here. I don't see how these trees are fundamentally different. From ARIA:

The accessibility tree and the DOM tree are parallel structures. The accessibility tree includes the user interface objects of the user agent and the objects of the document. Accessible objects are created in the accessibility tree for every DOM element that should be exposed to an assistive technology, either because it might fire an accessibility event or because it has a property, relationship or feature which needs to be exposed.

As I see it, all of that applies here. There are some subtle differences between platforms which we need to eliminate here because we're doing platform independent testing, but for the most part, the trees have a similar structure. While some browsers do have an internal concept of ignored nodes, I don't think those are exposed to any platform, though please correct me if I'm wrong.

In any case, on the call today, we discussed using a term like "implementation defined platform independent accessibility tree" or similar to clarify this a little. It's still a little wishy-washy, but it's probably the best we can do here for now.