I know you're not just looking for answers but a pointer to some better documentation, and I can't provide you with those, but:
why IVs shouldn't be considered secret
The least is considered secret, the least can be leaked and cause problems.
why the IV isn't required to be able to decrypt the file again
The IV is required to decrypt the file again. In the linked document's design the IV is actually the encryption key, which means it is known by the receiver, which is why it's not included. But that is just a special case that should never be reproduced.
Comments
I know you're not just looking for answers but a pointer to some better documentation, and I can't provide you with those, but:
The least is considered secret, the least can be leaked and cause problems.
The IV is required to decrypt the file again. In the linked document's design the IV is actually the encryption key, which means it is known by the receiver, which is why it's not included. But that is just a special case that should never be reproduced.