If I use the RdlViewer as mentioned in the wiki page, xlsx export creates an empty file.
The workaround to the issue is to set the DataSet again after calling the Rebuild method:
// This table needs to match the one you are setting
rdlView.Report.DataSets["DataSetNameInYourReport"].SetData(YourDataTable);
rdlView.Rebuild();
// Workaround
rdlView.Report.DataSets["DataSetNameInYourReport"].SetData(YourDataTable);