I don't think thats true, and here is why. When you look at an object you normally think about it and inside your mind you say "this is a rock", and that would be labeling it.

What happens if you see a rock and don't think about it though? You still saw it and you know its there but you never focused on it and so it doesn't have a label.

As for the otter, I would say it knows what food is and it knows what a rock is. If it didn't use any concepts it wouldn't be able to remeber what to do. If you ask yourself "what is a rock?", can you picture it in your head without using words? Yes, which means you must know what it is.

Just because we don't normally think of an object in a pure form without concepts doesn't mean we can't. We just can't talk to people about it.