It's a very standard defense-in-depth technique to put secrets between guard pages and only make the secret page readable when needed. That way any inadvertent access, be it programming error or exploit, simply causes a segfault, unless it's raced with a valid access (in a multithreaded or shm context) or the exploit explicitly changed the permission bits. Most memory disclosure vulnerabilities don't allow you to do that.

That being said any single password, when used, passes through so many layers and components that it's likely impossible to even just wipe the contaminated memory locations. But that's fine, the password database is opened for most of the browser's lifetime, any given password actively being used is a rare event in comparison.