Yes, you're right... "Sorcerer" was originally intended for a second Buckingham Nicks album.
As were "Rhiannon," "Landslide," "Monday Morning," "I'm So Afraid," and, if I remember correctly, "I Don't Want To Know."
When they joined Fleetwood Mac, "Sorcerer" was the track that Stevie has said that she was rooting for them to record, over "Rhiannon."
It may have been intended for a Buckingham Nicks album, but it was definitely offered to Fleetwood Mac before Stevie embarked on her solo career... but you're right, "first crack" isn't exactly accurate in that case.
Johnny Stew