Idle and light load is bad for degradation only because that's the most common scenario where the boosting algorith will actually go to the highest clocks. More loaded cores will have the CPU target lower clocks on all cores so that it actually can get the power for it and have the CPU be coolable, but if you're idle and then some task loads just a single core for a bit the CPU will boost it the highest it can. The voltage spikes from those boosts will cause local hotspots even if the CPU is cool overall