Your description is too reductive. You and I are also auto completes in some sense. See in order to complete a sentence well, you have to have a good model of a vast number of things including physics, psychology, linguistics, logical reasoning, socio economics, irony, sarcasm, arithmetic and many other things.
It is currently unknown how much of these the complexity of the models and training process will allow, but they have been surprising us in every step. You wouldn’t expect a “just auto complete” to figure out rules of arithmetic, but it did. You wouldn’t expect it to answer tricky questions involving theory of mind, but it does. You wouldn’t expect it to solve graduate level questions but it is able to.
So it’s a bit too rash to expect it to not understand rm -rf as humor, if you don’t know which model you will talk to.
The smaller ones, sure, are dumb. But even GPT 3 will not recommend you to rm -rf; definitely not GPT 4.