Even nowadays Excel is not fully SDI - if you do VBA or work with 3rd party add ins, there is a whole bunch of stuff that goes wrong since Excel is a SDI interface put on top of a MDI program.
This is actually weirder than that. Most MS Office programs are still MDI, but emulate being SDI. If you open up two different Excel files there is still only one copy of Excel actually running. This can cause all sorts of headaches for some addins when weird things happen. Notoriously if an error happens in a VBA addin a COM addin will stop responding to hooks.
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).