The Keypower is actually one of the PSUs shown to benefit from capacitor based tank circuits.
"One of" > you are even admiting the capacitor wont work on some setup, why? If the capacitor is so great why cant it work with any setup? Just because one guy got it to work on one setup it dont mean it will work on others. I do hope you realised by now why only some PSU can survive a crank with a small capacitor. If someone is using an inverter with a bad battery what will you tell them to use? a capacitor or battery?
If thats the sort of PSU he got then yes thats good for him if everything can be solved with a capacitor as you adviced. But Im not even gonna prove if that PSU can survive a crank or not in reality or theory because it already say on the thread it can operate down to 9V...most PSU just doesnt operate this low.
But dont you think these PSU designer should already be putting this capacitor in their circuit to survive the crank at the first place? Maybe its about time they realised this cranking problem, someone got to tell them that this where they been going wrong