Its even worse than that. They claim they can distinguish between background and Chernobyl radiation, but that is impossible to do with a radiation detector. What they erroneously assume is that background radiation is steady, and that all that changes while you walk around are due to Chernobyl. This demonstrates the ignorance of the author, because background radiation varies as you move around, and probably accounts for all of the measured radiation they show.
Evidence of their ignorance is even further shown in the schoolhouse, where they assume there is no background radiation.
Basically, they just arbitrarily decided what to call Chernobyl radiation. They have no clue.
Actually you can distinguish natural and man made isotopes with a gamma-ray spectrometer. There are various energy peaks associates with each radioelement that can be easily identified. For instance Cs-137 is what is usually measured to map areas of contamination from things like Chernobyl.
You are right about the background radiation varying everywhere, and with no scale these 'charts' are useless.