Nah they steel type so immune to poison :P

if only they didn't ask that question and stay within pokemon logic.

Lucario can still be poisoned by chocolate, it's not actual poison. Steel pokemon are only immune to poison toxins and things not food.

The only steel type pokemon that are completely immune to poisoning and poisonous substances lore wise are Aron, Larion and Aggron, Since they like to eat metals and poisonous metals taste tangy to them.
In Legends ZA there is a quest where you have fed a group Aron some scrap, but coat it with poison. She said "Don't try it with other steel other types as they aren't designed consume things that considered poisonous to most people."
This also means you can feed Skarmory and Corviknight substances that are poisonous to birds to harm them.

interesting <3