At no point since digital computers were invented was it inconceivable that we could continue prompt phrases by searching through similar phrases, near such prompts, in digitized text.
It was thought computationally infeasible, in just the same way that a harddrive would never be TBs, and so wouldnt be big enough.
But there's no magic here, and nothing suprising. It's presented by charaltans who have money to make as an achivement that it isnt. This isnt intelligence, and has nothing to do with it.
It can complete phrases by having seen TBs of similar ones no more suprisingly than google can. It is as limited as that process implies, namely, radically limited.
Likewise, it was never inconceivable that one could 'play' chess by searching through billions of chess permutations and selecting a simple high-value path. It was thought infeasible given, say, an abbacus to do it with.
We have never thought totally dumb solutions to these problems were unimaginable. They were just so dumb, that the only route to using them would be to somehow run a Ghz processor. Well, since we now have those, dumb partial solutions to dumb problems are possible.