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Late 1945 barely-more-than-prototype tech being used in 1943-44 isn't any more or less alt-hist fantasy than seeing a Black Prince on the frontlines. So is depicting depicting ultra-rare equipment as a regular frontline unit. And to me drawing the line at "if it was kinda sort of deployed somewhere at some point maybe then its fine" is artificial, to me using equipment that didn't exist (or practically didn't exist) is exactly that, and arguing whether on not it was "deployed" is splitting hairs.
And to answer why it was implemented? Well, they want to give UKF a KT-like heavy tank, and Black Prince loooks cool, mean and requires very little explenation, its a big tank with a big gun. And if it really bothers you that much, 17 pounders were used in Italy, Churchills were used in italy, just pretend its a field modification or something.
Look, I'd prefer the game to be historically accurate too, and I definitely agree that 1940-42 equipment would be way more interesting than repeating 43+ yet again. But COH was always full of bullshit, you may as well call it Company of Bullshit. The sooner you accept that, the sooner you can move on.