It means pearl and garnet were defeated at least once and had to regenerate before steven was born.

We know that regeneration still leaves someting on the outside like a scar such as pearl getting a sash after regenerating

Or garnet being bisected leaving her with a vertical line

So then that means that garnet was probably “killed” like this

And pearl was

But amethys (who loves shapeshiftin the most) doesn’t appear to have any major changes. She just had her hair grow and her undershirt got bigger.