At least I don't have any problems to accept the historical truth
This is the problem though. The historical truth is really not something black or white. The map, was reproduced in the late medievel ages, a time when Germany was expanding into the east, towards *pagan* Poland and Lithuania Commonwealth. They also wanted to get into Prussia. And what better way to legitimize this great crusade to convert and destroy Slavic and Baltic people, than to claim that this land is ancient German land.
Johannes Falkenberg, known as one of the first thinkers/writers to advocate genocide, would have been at the forefront of the crusade.
Look at what he had to say: "the Emperor has the right to slay even peaceful infidels simply because they are pagans (...). The Poles deserve death for defending infidels, and should be exterminated even more than the infidels; they should be deprived of their sovereignty and reduced to slavery."
It's not something new. Find me someone calling for the extermination of Germans, from the Polish side from that period, or from any other period (that is not a complete headcase in mental institute) This man was a dominican theologian! And widely accepted, from the looks of things.
Anyway, the map obviously has is its positive side too. It offers a look, albeit inaccurate, into the names/age of towns from the ancient period. But it could have too easily been fabricated/modified, what have you.