Adding the cost of bounds checking is not actually "zero cost" relative to a C default of not bounds checking. There's really no reason to speak in this misleading way except marketing.

Of course, bounds checking is worth paying a cost! As are many other common foot-guns like remembering to free memory. Which starts to raise the issue of when it would actually be necessary to reach for Rust instead of (arbitrary example) Java. Because a language with enforced bounds checking is not really the same kind of thing as C, and we've already had languages that are safer than C for decades.