You are right.
That linked file is a mix of OpenFormula syntax (ss:Formula=“of:=…”) stored in the old interim MS-Excel 2003 XML file format. That’s not going to fly. Didn’t even know that Excel allows such combination (if it was Excel that wrote the file?). In Excel 2013 (or newer) either save as OOXML .xlsx or as ODF .ods, in older Excel versions only save as OOXML .xlsx; best bet when saving in Excel is always OOXML .xlsx