We all know that Nero can't use magic, but she can speak. She could have easily told the story to Asta and the others about what happened. She could have easily told the truth to the current Wizard King, who could have easily found out everything, or maybe even turn back the first Wizard King and Nero to human form.
Nero was not able to speak before the elves re-incarnated.
Edit: Actually it wasnt until the shadow palace was summoned at Clover Castle. This is from the Wiki
> When Asta asks if she is Nero, she confirms and explains that she was originally human and could not speak as a bird until the Shadow Palace was summoned