But not used successfully, Bismarck, Tirpits, Scharnhorst and Gneisenau sank very few merchant ships.
Regardless, they were used in such roles. That is the point. Bismarck may not have sunk merchant ships but Scharnhorst and Gneisenau certainly did. Tonnage was around 115,000 tons for 22 ships. Not as much as U Boats but sink them they did and it was more Hitler's unfamiliarity with Navy operations, being a control freak in not giving his Navy the freedom to do their operations and worry about losing ships that minimized the danger rather than the Royal Navy itself.
The "Pocket Battleships" were no such thing, originally classed as "Armoured Ships" and later "Heavy Cruisers" the "Pocket Battleship" nomenclature was just allied propaganda, and they sank a lot less merchant ships than U-boats or auxiliary cruisers anyway.
At the era then, ships were classified by gun size than hull size. 11 inch guns were still classified as battleship weaponry and the heaviest of cruisers never went beyond 8". Hence they can still be considered battleships. Still quite effective at sinking ships, especially merchant hulls. Again, the fact that they did use them is more important here.
I still say that DE do not need a BB and it's inclusion goes well against the fluff.
What fluff? If that's your reason, then Eldar shouldn't get their Void Stalkers. They're as much a corsair fleet as their DE counterpart. Even the craftworld Eldar. They already are in the twilight of their existence. They basically hide and do that hit and runs. They do not need battleship scale ships in that case.
The question is this: if the DE's weak and sissy Eldar kin can have a battleship, why shouldn't the DE, who are vain enough to know they are their Eldar kins' betters, not have one of their own. If anything, their envy would make sure they have one for themselves.