A defective inference can accidentally end with a true conclusion. That does not make it a proof. A conclusion's truth and its being entailed by the premises are separate matters.
Example:
Every human is living.
Every horse is living.
Therefore some cats are living.
The conclusion is true in reality, but neither premise says anything about cats. It therefore does not follow from the two stated premises, even though an independent argument could establish it.
Fallacious arguments may hide their structural weakness behind a conclusion that happens to be correct.