The reason she knows the airship is there is because she used that airship to escape from the Empire when she was still a child, together with a woman. i think we still don't know about that woman name yet.
Randy also mention this, and he seem familiar with the woman too.

V!ta
My crack pot theory as to why the ship is on Purgatorio island.Well, i assumed it was the main protagonist (remember, they defected - escaped) Assuming it was before they became a (Diamond)-(Charon) thing??? Unless they are truly two different people - which i don't think is confirmed. (Memory loss)
Radiel266
Oh That's so cool. All this time... I've never really considered going back to Purgatorio Island... Maybe I should check it out.