Who are these "people" you speak of? It's possible to write software in low level languages that don't have these problems. Not a "non-zero" it might be possible, it can be done thoughtfully, and the popular notion it can't be done is backed only by incomplete anecdotes.
Should everything be written in low-level languages? No, that would be absurd. Is it a simple fact of life that not every person/team/organisation is capable of meeting certain standards of rigour? Yes. That's not to say anyone in the Bun team could not become sufficiently competent in the future. For whatever reason, current experience, incentives, and personal motivations did not make for a conducive environment to make Bun watertight in Zig.
Zig does help you. Array slices, explicit nullability of pointers, defer errdefer, explicit allocators, built-in leak detection, bounds checks, overflow detection, the list goes on. If you need to play around on that side of the fence, Zig gives you a lot to make sure you don't mess it up. If we were talking about C I'd give you your flowers, but we're not. The most common issues and vulnerabilities that crop in C from manually managing memory are strongly mitigated by a quarter of that list.
The good news here is that we have more than just anecdotes to support this, we have empirical evidence.