Thanks for providing us template file.
I have evaluated your scenario/case a but using your template file and following sample code. Well, if you could open your template HTML into MS Excel manually, you will also see those long numbers are changed to scientific notations. This is MS Excel’s behavior and nothing to do with Aspose.Cells APIs. See the screenshot for your reference:
If you got to display full long numbers, you have to apply number formatting or custom formatting as “0”, then all those cells will display long numbers fully.
HTMLLoadOptions opts = new HTMLLoadOptions(LoadFormat.HTML);
Workbook workbook = new Workbook("F:\\Files\\CAD=AOApgmov94Z_bZ9txfSP-YtsM1x6FYu-CZAG7sX4sa5nCYQ@mail.gmail.com_1\\CompleteEmail.html", opts);
String output = "F:\\Files\\CAD=AOApgmov94Z_bZ9txfSP-YtsM1x6FYu-CZAG7sX4sa5nCYQ@mail.gmail.com_1\\out1.xlsx";
Hope, this helps a bit.