That is assuming that at least one of her parents is a citizen of Bangladesh by birth, not descent. I'm not sure if that was ever definitely confirmed. If her parents had citizenship by descent, then her birth would also have to be registered at the Bangladesh consulate.
"Shamima Begum’s father, Ahmad Ali, a Bangladeshi national who has returned to his ancestral town of Sunamganj after emigrating to Britain in the mid-1970s, said the family was working with an English solicitor to weigh the legal options left in his daughter’s case."
Is all I can find apart from the previous article I linked which contain comments by the father.
https://www.benarnews.org/english/news/bengali/citizenship-issue-02112020162921.html