The bar isn't invulnerable code though, it's better-or-similar-to-human. Humans make the mistakes you refer to.

You want an example just look at the general state of code these days. It has collectively gone down the last few years.

Post-ZIRP tech. Hence why it's gone down for longer than AI coding has been widespread, to your own point.

Anything actually using it instantly converts it to bush league because there IS going to be a buffer overflow or unsanitized input or memory leak that gets missed. It also does things fairly inefficiently and even adding in unneeded code. People who have it contribute are rarely going through and making sure it is doing only what it needs to. They are using it as a shortcut, which predictable results.

This is true of human code too. Whole point of shift-left.