We used to have long discussions about the medieval warm period here. Most of the warmers deny the existence of the medieval warm period, and as you say denial is more than a river in Egypt.
When the warmers draw their temperature graphs, they almost always ignore the medieval warm period, and sometimes even start from the trough of the little ice age.
The lesson to be learned from the medieval warm period and the mayans, and indeed past warm periods such as the Roman warm period and the minoan warm period, is that so-called global warming is a regular cyclic phenomenon on planet Earth.