I am interested in exactly the same thing. Our migration to Forge is only blocked by this feature, as pointed out in the post you linked.
Now draw.io is officially Forge but they open things in fullscreen. How do they do that? When I look under the hood via the Inspector, I can see that the macro view and editor etc. are still a good ol’ connect iframe serving from the draw.io connect backend as if nothing has happened.
Maybe the whole “draw.io is Forge” thing is just marketing and its Forge manifest just contains connectModules for the whole UI? Would be great if somebody could explain this!