The Supreme Court has ruled she can't come back to UK to fight her citizenship case.
for now
Correct. The court has ruled that she can fight her case, if there is a way for her to fight it. But she cannot return to UK to do so. And she cannot leave the camp to do so, because without any citizenship, there is nowhere for her to go.
But the refugees camp will not allow access for her lawyers, and the court does not provide a means for her to fight her case. So she is effectively denied the opportunity to present her case.
Lord Reed said the appropriate answer was not to force the government to bring Ms Begum back to the UK - but to pause her legal fight over citizenship until she is in a safer position to take part in her appeal.
https://www.bbc.com/news/uk-56209007
In addition, the court does not, and cannot overrule the assessment of the government, about whether she is a risk to national security, because they cannot see the information that the government has. Unless there is overwhelming evidence to refute the government's assessment, and that cannot be presented because SB is denied the opportunity to do so.
Moreover, the government's assessed risk is limited to general statements such as, "she has aligned herself to ISIS." There has been no credible threat mentioned by the government as to the risk posed by SB.