As we now know from the Climategate Emails, this band saw the Medieval Warm Period as an enormous obstacle in their mission of spreading the word about global warming. If temperatures were warmer 1,000 years ago than today, the Climategate Emails explain in detail, their message that we now live in the warmest of all possible times would be undermined. As put by one band member, a Briton named Folland at the Hadley Centre, a Medieval Warm Period "dilutes the message rather significantly."
Fallacy: conspiracy theory