It's possible that the alternator doesn't have a diode bridge (bridge rectifier) to convert the AC to DC (or any rectifier for that matter). I assume it uses the alternator to power AC items and the generator to power DC items in the plane.
It doesn't make sense why they wouldn't just add a diode bridge to the alternator. Like some people have said it's possible the starter is also the generator and therefore you don't need a diode bridge. Still a little weird.