The actual report text identifies the uploaded database as "NUMIDENT".
A quick shufti turned up https://aad.archives.gov/aad/series-description.jsp?s=5057 which states that "NUMIDENT" includes things like "mother's maiden name". Other sources imply that the signatures from SSN application forms (form SS-5) are stored here.
> In addition to SSNs, the database reportedly includes Americans’ place and date of birth, work permit status, and parents’ names
This is quite a bit more information than just a number.