Well... 

"Is this not the same gun as on the pocket battleships of the Deutschland"

No.

"which the British cruisers at the Battle of the River Plate survived"

Hitler was quite right when he said that Admiral Graf Spee should have won the battle. She lost because of errors made by captain Langsdorff. Graf Spee outgunned all her opponents, having 11 inch main guns, to Exeters 8 inch and Ajax and Achilles 6 inch guns.

Langsdorff should have concentrated the fire on Exeter first before Admiral Graf Spee came into the range of the light cruisers.

Instead, Langsdorff split his fire between the three targets, with the result that although Exeter was severely damaged and forced to withdraw, the other ships got into range and scored 20 hits on Admiral Graf Spee, including on the food stores and bakeries.