What you describe sounds nothing like most, safety-critical development I've heard of. Whereas, I've heard the other person's story countless times when I studied high-assurance systems. Very slow, top-down, process-heavy, paperwork-heavy, and outdated tools.
On the other hand, it sounds like the company you mentioned is worth imitating where possible. They sound awesome. Are you allowed to name them? Is there any writeup on how they balanced velocity and regukatory approval?