It's outlandish that the Kormoran didn't know the secret callsign of the ship they were imitating. How were they unaware? Clumsiness? Couldn't the Kriegsmarine inform them?
Please see the question in my title, and the emboldened sentence underneath. I quote Wikipedia.
During the exchanges and distress signal, Sydney positioned herself just off the raider's starboard beam on a parallel course, approximately 1,300 metres (4,300 ft) from Kormoran. The cruiser may or may not have been at action stations: the main guns and port torpedo launcher were trained on Kormoran and her Walrus scout plane had been readied for launch, prompting Detmers to prepare to engage Sydney, but her 4-inch (100 mm) guns were unmanned, and personnel were standing on the upper deck. During her manoeuvre, Sydney appeared to signal "IK" (the short-form for "You should prepare for a cyclone, hurricane, or typhoon"), which Kormoran did not respond to, as from their perspective, such a signal did not make sense. The Germans were unaware that the letters were the interior of the real Straat Malakka's secret callsign, "IIKP": to verify her identity, the ship had to signal back the outer letters. The aircraft was shut down by 17:25, and the catapult swung into the storage position; the two ships were too close for a safe launch.