Definitely not true in my opinion, my Eartheaters love the tank around 27 Celcius (80-81F), but all do fine in slightly lower temperatures during winter; the Gymnogeo's moreso.
That's the problem with common names. So many fish get lumped under "eartheater" when it's really several very different genera of fish. Most Gymnogeophagus like it in the low to mid 70's, with a 2-3 month cool down.