Of course, I immediately found the answer after posting. In Chapter 9 of the Scripting Guide:
"Note: To use the Excel Wizard, you must specify the Excel Wizard option in the argument as in Open( "$SAMPLE_IMPORT_DATA/Team Results.xlsx", "Excel Wizard" );."
Instead of deleting the post, I figured I'd leave the answer here if anyone else ever needs it.
-- Cameron Willden