Is there a way to export an Account Summary Report to a csv or Excel file?

No. Account Summary is one of the old Quicken reports which came from the predecessor product, Quicken Essentials. The developers have not yet replicated this report in the new reports engine -- all the reports you can generate with "New Report" on the Reports menu -- that's been under development.

You can select all the text in the Account Summary report, copy, then open Excel and paste. It'll require some clean-up, but all the data pastes in in a usable form.