His point is that you're not forced to do that. And anyhow, that doesn't solve the issue since you can bungle the creation of the slice with the wrong offset or length.
Not bungle in the sense of overflowing the underlying buffer, but overflowing the logical buffer that is contained within it, i.e. getting the wrong slice.
However, you can't bungle the creation of a slice in rust without using explicitly marked unsafe code.