While I understand what you are trying to say with your analogy I think it shows a misunderstanding of how they work. If you meant a very good autocomplete then yes that's exactly what they are. However, predicting the next word in an autocomplete can be naive or rather involved.

> not really useful for anything other than a VERY good spell checker.

Try GitHub code pilot. Try asking it to summarize something you know nothing about and then tell it that you need the explanation to be more simple several times. How can you compare this to a spell checker?