Yes, kids learn a lot by observing others. Your 5 year did not talk to a Google device before he saw someone else do it. Kids learn just from watching you, its still teaching and learning. Saying OK Google to electronic devices is not innate in humans.
I guess "teaching" can be done passively, though I feel like the context of my comment made it clear that I meant "teach" as a purposeful action. Maybe you can read it as, "we never instructed him to", if being pedantic about the word "teach" is preventing this conversation from moving forward.
