This isn't MDI, it is just using the same process for multiple toplevel windows. The defining characteristic of MDI is that the toplevel windows has subwindows

This isn't MDI, it is just using the same process for multiple toplevel windows. The defining characteristic of MDI is that the toplevel windows has subwindows inside that you can move and resize around (almost) like toplevel windows. Visual Studio's MFC wizard actually has an option for this behavior called "Multiple top-level documents" (which is separate from "Single document" - aka SDI - and "Multiple documents" - aka MDI).