i just think lydiard was a laid back guy. When he said easy aerobic run, people interpretted it as on the easy side of the aerobic spectrum. I think he was just meaning an aerobic run, which if done aerobically, should be easy.
And the easy was probably compared to normal athletes idea of a run, which was run till the wheels fall off then do some strides….
To be honest a Lactate Threshold run would be considered aerobic, and it is defiantly easy compared to anerobic sharpening.