Never quite fully understood this as well.
V2500 - Engine oil is first cooled via the Air/Oil Hear Exchanger before entering the Fuel/Oil Heat Exchanger for another round of heat exchanger. The fuel that cools the engine oil in the Fuel/Oil Heat Exchanger is part of a rather complex system (refer to FCOM diagram) where it also routed to cool the IDG oil.
Based on this I infer that if there’s something wrong with IDG leading to high IDG oil temperature, it will cause the fuel that comes through to cool it to warm up significantly enough to be inefficient to cool the engine oil effectively when it eventually reaches the Fuel/Oil Heat Exchanger. Hence the procedure to increase fuel flow if possible or reduce IDG loads.