Was the code free to copy and use? No.
Was the code publicly published? Yes.
Was it open source? Unfortunately the term HAS been overloaded but the answers above are more relevant.
Were people able to lay eyes and audit the code? Yes.
Were people incentivized to do so? Considering it's popularity and use by individuals, yes.
Could it be more scrutinised if it had a more permissive license? Probably. Passport was a fork of sorts, but it was forked well before the bug was introduced.
That much more? No.
Is it possible that it had a lot of eyes on it yet no one saw it? Yes.
Was it a stupid bug in hindsight? yes. 😢
Was the bug there visible for everyone to see? Yes.
