The singer-songwriter wrote on Twitter that Fiorentino - who joined the group in 2010 - had revealed that she was one of the previously-unknown children on the sleeve.
Corgan said: "Just found out the weirdest news: our bass player Nicole just admitted she is one of the girls on the cover of Siamese Dream. She said she didn't want us to know because she thought maybe we wouldn't let her be in the band."
However, an assistant to cover photographer Melodie McDaniel reportedly told Rolling Stone that the image was shot especially for the sleeve. Fiorentino would have been 14 when the photograph was taken and both girls on the sleeve appear to be much younger.
Fiorentino retweeted the story from Rolling Stone, appearing to debunk Corgan's claim.