You're right that bounds checked arrays do nothing at all for existing code. But they can be added incrementally to an existing code base, as a normal part of working on the code.
In that aspect it's like when prototypes were added to C. Nothing changed for existing code, but prototypes are so advantageous people would retrofit existing code incrementally when doing routine maintenance.
I presume prototypes could be added to transform much of existing code without compatibility problems (ABI nor API), which is a clear advantage compared to checked arrays. Checked arrays might worth it regardless, but it's not applicable in most of the article's use cases.
