Is this not an overstatement? How does a person understand code if they write so much of it with AI?

The same way a tailor understands stitches even if a sewing machine creates most of the stitches a tailor will ever stitch. Because understanding code is tangential to writing it. Which isn’t to say that writing it doesn’t help solidify knowledge and certainly plenty of people learn by doing and there may even be skills that atrophy as a result of not writing code in the same way that skills for writing assembly have atrophied with the use of higher level languages. But ultimately it is possible to understand some code without having written most of it by hand.

Because I tell AI exactly what and very often how to write the code to avoid sub-optimal solutions AI so keen to propose if not properly directed.

As for autocompletion, not sure about every tool but CLion and other IDEs I have from JetBrains are genius. Yes they can autocomplete multiple lines of code with a single keystrokes and no I do not really want to write it myself as it mostly boilerplate code I've written many times and autocompletion just predicts it.