But iocane powder on the other hand...

Corpses are immune.

It is cyanide. But (1) cyanide does not accumulate in the tissues over time. If you were to have a constant small drip of cyanide injected into your veins, as long as your body could dispose of it at the same rate that you recieved it, you wouldn’t die. Ie, you arent going to see gradual onset cyanide poisoning over years of eating apple seeds. And (2) apple seeds contain a compound that makes cyanide when it is digested. But the compound is inside the apple seeds. So unless you are really focused on grinding the seeds to a paste, you won’t actually have the compound reach your digestive system. It will stay contained within the hard shell of the seed, and you’ll just poop out the whole seed. Which is kind of the whole point of fruit-bearing plants in the first place. The plant wants animals to eat its fruit, then walk away with its seeds in their bellies. Then they poop out the seeds somewhere else, spreading the plant’s lineage geographically, and providing the seed with some good fertilizer to boot. The seeds, and the dangerous compound inside them, don’t just haphazardly pass through your digestive system - they are specifically designed to pass through animal digestive systems unharmed. And (3) the amount of cyanide you can get from apple seeds is quite small. Unless you are actively, like, making apple seed paste and eating it in bulk, you'll be fine.