I don't think anyone is using "autocomplete" in the way your phone does it. But it is shorthand for a much more sophisticated version that is built in similar ideas. And autoregressive models certainly have that in their core structure. But people are lazy and neither want to say a lot when few words work nor will they read long comments, even if more accurate

I want to clarify, most people are using "autocomplete" specifically to differentiate from how we humans operate. Sure, there is an autocomplete aspect, but it's not the core nor anywhere near the full story.