Not an exploit at all, the sum of 12.5m food per hr is a rather conservative number from the amount of food gained from holding 10 lvl 10 barbs. Of course, if you take more then 8-9 you cripple your ability to wage war, but the number is still between 9.5m and 20m food per hr passive food production.

Now, if you actually have a hero above 225 atk or so, you can still effectively farm lvl 10 barbs with few losses. I dunno if that'd be concidered a exploit or not tho.