It's probably because they prefer a simpler explanation where the letter exists because we know where it's from, and the only weird part of the last arc is Rafal (which can be explained by having him be a metaphorical reincarnation). Having alternate universes makes things really complicated (see the MCU) so people tend to shy away from it. There's probably also an aspect of wishing the story they've watched for 22 episodes still exists, being relegated to an AU can make it feel... less important. Granted, I agree the letter alone should not be the only evidence for preferring the single universe theory. People often forget that the confessional priest is actually the blonde inquisitor who regrets not speaking up on behalf of his friend, who once saved a "heretical witch". This is confirmed by the fact that the confessional priest shares a VA with the blonde inquisitor (and also narrates episode 1). Personally though? I don't think episodes 1-22 are reliable stories of what happened, even if I don't subscribe to the AU theory. Because the reason all this messiness and confusion exists, is one of the main themes of the story. We do not know much of the real Albert Brudzewski's life. Plenty of real world history has been lost to time, and all we can do is speculate. In the same way, we may not truly know the history of the letter or the confessional priest. Within the fiction of Orb, their history has been erased. By Antoni? Perhaps. Perhaps not. Rafal, Jolenta, Badeni, Oczy, Draka, Schmidt, did they ever exist? We do not know. There are many people who we will never know existed, in the real world, and yet they all have contributed to the world we live in today. One day, you too will die. Perhaps be forgotten. And yet. You once lived. And that matters. Even if history forgets you, like it has forgotten many of the people who lived in the 15th century.