Apple seeds release the cyanide (HCN) upon metabolizing the cyanide/sugar compound found within apple (and other Prunus sp.) seeds. Two (2) things need to happen, 1) the seed (pip) hull needs to be chewed or ground up so that 2) digestion of the compound can occur.
Apple seeds that have been cooked are considerably safer than raw.
While the potential for cyanide poisoning is there, it seems that the probability is pretty low.
My hens have consumed lots of apples over time with no ill affects. I don't think they actually chewed the seeds but I am kind of wondering if grit in the crop would be coarse enough to breech the seed hull. Or does the seed simply pass through the gut. A scat analysis might be interesting.
But my Mom always said to never eat the seeds because a tree would sprout and grow in your gut.