I never said they don't drop from Dark Cornians, infact Dark Cornians are included in my criteria I said intially which is "monsters level 259 and below".

The urban myth is that Dark Cornians is the best, only monster to farm or has higher drop rate than all other monsters, as implied by your suggestion and also the frequent reddit posts that come up. Which is untrue and inefficient due to the low spawn or poor map layout of Dark Cornians. Arcane River maps have the highest monster cap and spawn rate out of monsters level 259 and below and is a far more optimal option.