Sometimes, factual errors are actually deliberate in order to improve gameplay. For example, accurate architecture may take a back seat to level design. Weapons may react in completely unrealistic ways, but it is understandable because in real life, people didn't fight with controllers. People go much faster than they should, distances are shortened, there is either too much or too little variety because having too much of the same thing is boring and having variation is every small detail is too expensive. All that is not just excusable, it is actually good design, it is a game, not a history lesson. Sometimes, though it is more relevant in movies, there is a deliberate inaccuracy just because that's what people expect and doing it differently would make the audience focus on an unimportant detail or mess with the pacing. And sometimes, it is just to save money that is better spend elsewhere.
I don't think having period accurate pigs is any of these, so for me, it is a mistake, no more, no less. Not the worst, but it deserves a "bug report".