They can definitely go out of sync, particularly if something that isn't the editor or the AI changes the code (e.g. running shell commands or opening the file in a different editor and making changes there). I've had a whole load of issues with VSCode where there's been spurious edits all over the place that show up again even if I try and revert them, because every time the AI makes an unrelated edit, VSCode tries to reset the file to the version it thinks exists and then play the AI's edits on top.
I think this statement is misguided, and potentially comes from a lack of experience in getting AI coders to produce quality.
Proper engineering does not come about from the tools you use or how you use them. Proper engineering has always come from thought, and reasoning, it never was about the act of coding. It always was about the systems thinking and expressing the goals and desires that matched the requirements.
IDEs were never needed to properly engineer and in the days of AI will become increasingly less important.
Tools for planning, reviewing, and commenting on code are the future. The necessity to edit actual code is coming to an end.