Offcanvas with body scrolling - confusing/illogical focus behaviour and concept

The idea with the offcanvas is that it's conceptually a modal dialog that just happens to be styled differently. However, the examples with body scrolling https://getbootstrap.com/docs/5.0/components/offcanvas/#backdrop break this concept - focus isn't moved to the offcanvas when opened, and not maintained in the offcanvas.

What's the intended use case here? Making this a non-modal offcanvas dialog? (that can be open while the main page is also still active)

If so, this needs a lot more work (treating the offcanvas essentially as a completely separate page/zone, and implementing keyboard interactions (e.g. using Ctrl+F6 or similar) for the user to move focus from the page to the offcanvas and back again, and then within those, keep the focus cycle clean.

Particularly puzzled by the backdrop+body scrolling example...what use case is this trying to address?