Explanation
No. The Gutenberg Bible, produced in Mainz in the mid-1450s, was not the world's first printed book. A surviving Chinese printed copy of the Diamond Sutra bears a date corresponding to 11 May 868.
The British Library's International Dunhuang Programme describes that scroll as woodblock printing on seven joined sheets of paper. The Library of Congress identifies Gutenberg's Bible more specifically as the first great book printed in Western Europe from movable metal type.
These are different printing methods and different historical milestones. The Diamond Sutra provides a documented earlier example; its status as the earliest dated printed book does not establish that printing itself began in 868.
Key takeaway
Gutenberg's Bible marks a major European printing milestone, not the beginning of printed books worldwide.