It’s not synonym, cult means that the “correct” way to write it is “criatura”. Is in the index because the RAE acknowledge that “creatuta” exists as a word.
cult. just means “creatura” is closer to Latin creatūra. Also, as far as I know, the RAE doesn’t list any misspellings. I’d take them acknowledging it exists as a word to be enough for it to count as “proper” Spanish.
I didn’t say is a misspelling. There’s a reason why the RAE page have all the definitions and synonyms in the “criatura” page (that dosen’t include “creatura”) while “creatura” only links to the correct spelling “criatura”.