The English courts have decided that, under Bangladeshi law people born of Bangladeshi parents are automatically Bangladeshi nationals until the age of 21. English courts decide questions of foreign law as questions of fact; for the purposes of deciding whether or not she is a Bangladeshi citizen insafar as that's relevant to the application of English law, no decision by a Bangladeshi court is required, and comments by Bangladeshi politicians are irrelevant.
It's not hypothetical citizenship; it's legal citizenship, as is all citizenship by birth. A child born in secret in the UK doesn't know they're a UK citizen, and neither does the state, but they are.
And all citizenship is racist insofar as it descriminates against peole based on their nationality!
I'm willing to bet £50 to the server fund that, if English courts ultimately decide that the Home Secretary acted unlawfully, it won't be on the basis that her being a Bangladeshi citizen (under the application of Bangladeshi law) is discriminatory. (It'll be on the basis of deviation from the practice of extra-territorial application of articles 2 and 3 ECHR.)
Honestly, that's the weakest part of her case; as you'll see from the judgement in the Court of Appeal (
https://www.judiciary.uk/wp-content/uploads/2020/07/WP-Begum-Judgment-NCN.pdf), she sought judicial review of the decision not to grant her Leave to Enter (to pursue her case), and two of the three preliminary findings of the SIAC - it's finding that she wasn't made stateless is the only thing she didn't seek to judicially review.