This is just pure nitpicking if you ask me. We can as well say, why "War Machine" existed in vCoH, because it also does look kinda fictional. I will repeat myself again, lets just sum up what kind of fiction was allowed for CoH across both games:
1) Misrepresentation of units
2) Some fictional abilities on units
3) Rare or super rare used equipment representation
4) Believable abilities made to fit gameplay
5) Fictional, but belivable, ammunition on a small scale (grenades mostly)
6) Pure gameplay abilities which-in the scope of represented factions (vCoH USA "War Machine")
7) Wrong time periods
8) Wrong availability of rare equipment
Where would you put Black Prince? It wasn't finished, it wasn't used, it wasn't put into a production. Literally 6 were made and all 6 were prototypes. Besides the fact that BP looks like a ww2 tank, it doesn't fit into any category CoH previously allowed itself to mess with.
Its plenty close enough to option number 3 imo....
Sturmtiger was extremely rare, and yet during multiple periods of the coh2 meta u would see one EVERY single game. Yet most WW2 vets wouldnt even recognize one if u showed them a picture
Sure ST was actually used in ww2, not disputing that. But as long as BP isnt dominating the meta i dont see the problem
"This existed on the front AT LEAST once" is a silly line to draw imo. Especially in a franchise that has warped that line all over the place