Atheists, how do you explain God correctly knowing that Pig was unclean back in the day?

I explain it as a false umption. God didn't "know" that pigs were "unclean." You are incorrectly uming that there was a supernatural being in charge of writing the book. The more realistic explanation is that the (very human) author of the Bible wrote it, probably because he saw his friends and family get sick and die after they ate uncooked pork. All that it proves is that people have the ability to write down their experiences to p on to future generations.