Nelson Mandela died in 1991?

I came across a snippet of a South African educational book published on 1 October 1991 and mentions that Nelson Mandela died on 23 July 1991.

The snippet found on Google Books reads:

Yet, words like reconciliation and the new South Africa simply were not long enough to bridge the vast differences between blacks and whites. The chaos that erupted in the ranks of the ANC when Nelson Mandela died on the 23rd of July 1991 brought the January 29th 1991 Inkatha-ANC peace accord to nothing. It lacked one thing. It lacked the last realisation.

Book Title: English Alive, 1990: writings from High Schools in Southern Africa

Authors: Kathleen Heugh, Anita Kennet

Date of Publication: 1 Oct 1991

Page No.: 54

Publisher: Claremont: Western Cape Branch of the South African Council for English Education, 1991

ISBN: 0620151862 9780620151863

OCLC Number: 123781891

Source: Google Books

Source:- http://www.timelineshift.com/2016/08/south-african-book-from-1991-nelson-mandela-died-on-23-july-1991.html