Logic Pro does not switch to Realtime Bounce mode while exporting sessions that include external instrument tracks, even though the manual says that it will. As a result, the external instrument tracks are not included in the export.
The manual's statement is an error in the documentation. To export a session that includes external instruments to XML, perform a realtime bounce of the external instrument tracks before you export to XML. Be sure to add the bounced tracks to the Arrange window before exporting the session.
This document will be updated as more information becomes available.