If a robot assembles cars at lightning speed... but occasionally misaligns a bolt, and your only safeguard is a visual inspection afterward, some defects will roll off the assembly line. Human coders prevent many bugs by thinking during assembly.
IMHO more rigorous test automation (including fuzzing and related techniques) is needed. Actually that holds whether AI is involved or not, but probably more so if it is.
I'm far from an AI true believer but come on -- human coders write bugs, tons and tons of bugs. According to Peopleware, software has "an average defect density of one to three defects per hundred lines of code"!