Its shape, growth during the firing and penetration of glaze down into the crack demonstrates it preexisted firing (happened during the drying).
Clay Cracking During Drying
The best way to avoid drying cracks when making ceramics or pottery is to avoid doing the things that cause it. Do not just blame the clay, anything can technically be dried.
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.