You're certainly right about the destructive power of the muzzle blast from big guns, parabellum. On most capital ships, if the ship's float plane was still on its catapault, it would be damaged (if not, for all intents and purposes, destroyed) if the main armament was fired.
The float plane would often as not be launched before the main turrents came into operation - after all, spotting fall of shot was one of the aircraft's major tasks after identifying the enemy aircraft.
Sydney was a little bit different in that it aircraft was midships and possibly far enough away from the forrard turrets to survive the muzzle blast when they were fired. (Perhaps someone more expert than I am on things naval/nautical could comment on that point.)
Sydney also had torpedo tubes, and the torpedo crews at their action stations certainly had to be on the exposed deck. Detmers' report mentions that his anti aircraft cannon and guns virtually wiped them (the torpedo crews) out as the two ships passed at close quarters.
Anyone who saw 'Saving Private Ryan' would have a layman's idea of what 20mm cannon can do to 'soft skinned targets' - i.e., men. Many of Kormoran's anti aircraft cannon were fast firing 37mm, more or less the equivalent of the Allies' 40mm Bofors, which would do immense damage at such close range.
It doesn't bear thinking about what it would have been like for anyoone unfortunate enough to be on Sydney's open deck or in any compartment not protected by at least a inch or two of armour plate.
The question that continues to plague me is: what in the hell happened to cause the captain - (however inexperienced) - of a major warship to put his ship in such a situation? Surely to God it wasn't just his perceived need to get things sorted and the ship identified before last light? A situation like that would have been covered many times in exercises since he'd been a Midshipman.