Hello,
that is an interesting observation. Technically, recording a MIDI-track directly into an Audio track is not possible at the moment. The only way to do this would be to route the output of your MIDI-track to your interface and connect the output to an input of the interface. Another way would be to record the output of the speakers using the internal microphone of the iPad (which would yield somewhat bad quality).
As well, recording the output of other apps is not possible. This is what Audiobus (http://audiob.us/) promises, which Cubasis will support with the upcoming update that is waiting for approval in the App Store at the moment.
If you have further questions let me know.
Kind regards,
Frieder