The vulnerability arises from sanitization occurring before jQuery's DOM manipulation. The core issue is that Jupyter's sanitize_html function (or equivalent) failed to account for how jQuery would reinterpret invalid HTML remnants after sanitization. This allowed malicious content to be reconstructed into executable JavaScript during jQuery's parsing phase. The high confidence stems from the described attack vector (sanitization bypass via post-sanitization DOM manipulation) and the logical location of such sanitization logic in Jupyter's security-related JavaScript files.