If the main criterium is "has this equipment been used in WW2?", you're not aiming at authenticity either.
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.
At this point, it would be fine to use a normal Churchill (hek, even a Sherman) and just slap Black Prince stats on top of it so it can defeat a Panther. I am sure most people would not be fine with that.
And it would have been perfectly fine with-in the scope of how CoH gameplay mechanics works. Aside from balance\faction reasoning and some commons sense, kubelwagen could be made into a TD, with an ability to fire its MG into some tank weakspot or something