。
She didn't promise that she'd come here again.
Literal
She [topic-は] again here [at-に] come [quotative-と][contrast-は] did-not-promise.
The combination とは layers the quotative と (marking 'again here come' as the content of the promise) with contrastive は ('as for that promise, no'). とは effectively narrows the negation onto the embedded clause: 'it's not that promise specifically that she made.' This is subtle but common in careful written Japanese — とは is more emphatic than と alone in negative contexts. The plain past 約束しなかった keeps the register neutral-plain.