I'm certain that the maintainers of Bun have excellent understanding of their codebase. What makes you think that they don't? They wrote the code in the first place. They know the architecture. They know what pieces do what functions.
This is a massive codebase created within a week or so ago, no one can say what it will do till it does.
I still think it's mad, but not quite as mad as you might first think from the headlines.
Who wants to review a brand new codebase for free?
With zero reviews and no one even reading the code, there's zero confidence. For all we know, the new Bun could contain a change that causes JS file reads to return a novel for a very specific filename, and the tests people keep pointing to, intended to downplay the malpractice, would never catch it. Tests cannot cover every contrived scenario that normal human beings would never think of writing.