It was nearly 24 years ago, so there's a window of 5-6 years they could have been born after following the last referendum and be eligible to vote in this one.

You're also oversimplifying what the preamble was - the recognition of indigenous Australians was one point among many. Full text here https://parlinfo.aph.gov.au/parlInfo/download/legislation/bills/r900_first-reps/toc_pdf/99160B01.pdf;fileType=application%2Fpdf