Indeed, though as I understand it, Lumina still bundles a file manager. How necessary is this? I don't have a good understanding of how e.g. Nautilus integrates into things like the file save dialog for Chrome.
The alternative would be to bundle a terminal, I think - a window manager that doesn't offer a way to run programs, is a window manager that fails as an interface.
In X11, there is a big difference between a "window manager" and a "desktop environment".
That feels like basic functionality to me. You need a way to open programs/files at least.
Yes, but why does the WM need to provide it? Everyone has preferences - pick your own.
As has been pointed out already, the headlined item is named Lumina Desktop Environment, not Lumina window manager.