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.
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.
The window manager is Fluxbox.
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".
Is Lumina different from say Xfce in this regard ?