Was she a dual national? As I understand it she didn't hold any other citizenship but was apparently eligible to have another.
According to UK legal scholars, she was, as you said, entitled to Bangladeshi citizenship, because at least one of her parents were Bangladeshi, even though she'd never visited Bangladesh. But Bangladesh have denied her entitlement, and have stated that they would refuse her application, if she applied.
Obviously UK has no jurisdiction over Bangladesh, and cannot force another country to accept an application for citizenship. And if UK have the authority to strip someone of their existing citizenship, who was born in UK, and lived her whole life in UK (on whatever grounds they think fit), then Bangladesh certainly has the authority to refuse an application on whatever grounds they think fit, especially to someone who has never visited the country, and has never demonstrated any desire to be there.
Therefore the whole premise of UK's decision is based on her being entitled to another citizenship elsewhere, over which UK has no control. The other country have denied her entitlement, and stated that any application would be denied.
Therefore she has been rendered stateless.
What next? UK refusing asylum seekers applications on the grounds that they would be entitled to asylum somewhere else?