The Cube View setting [Landscape] (True/False) applies only to PDF (report view), and not to Excel.
There is an existing enhancement request TFS20360 for this to be added to the Excel export for cube views. I suggest logging a ticket with support with your customer name, and asking for your customer name to be added to this TFS number.
I don't think there is a code-workaround for this at the moment. The only way to do this would be to implement your own export-to-Excel button on a dashboard , which would run an Extender rule which uses DevExpress (or similar) APIs to create the Excel workbook extract.
Regards
Chris
