Why is it said that the destruction of the Library of Alexandria set humanity back hundreds of years?

For the same reason that people say nonsense like “if Aristotle had not written about physics, humanity would have advanced faster in science.” In other words: Eurocentrism of the most vulgar and rude. Whether it was the burning of this library in Roman times (it was later rebuilt, and a large part of the collection of copies stored elsewhere could be recovered) or the one it suffered during the Arab conquest (idem), this did not affect the development of science on the other side of Eurasia, specifically in India (where pure mathematics was for centuries a little more advanced than in Europe and the Middle East) or in China (where mechanical engineering was a little more advanced).

Do you want to know that 3 historical events actually set humanity back several centuries in its development?

1. Prohibition of power looms in Song China. Yes, the Chinese had created mechanical looms powered by hydraulic power in the 11th century. They were one step away from an industrial revolution…but the mandarins were afraid that the proto-industrialists were acquiring too much money and power, so they were banned…and soon after unsustainable court spending led to abuse in printing banknotes created thus a hyper inflation which, added to the enormous corruption, made the government not react in time to the Mongol threat…
2. The Mongol expansion: 10% of humanity (yes, the entire planet’s population, I’m not exaggerating) perished in Genghis Khan’s wars of conquest. Afghanistan lost 90% of its urban population and with it almost all of its literate people and what was left of its written heritage (which had already suffered great destruction during the Arab conquest). Much of Central Asia saw the entire population exterminated, hence they stopped speaking Indo-Iranian languages ​​in places like Tajikistan or Kazakhstan, being replaced by speakers of Turkic-Mongolian languages ​​that live there today. China lost half its population and it took almost a century to recover,
3. The cancellation of the “Star Expeditions”. The imperial eunuch Zheng He, admiral of the fleet of the 3 treasures, led a series of voyages of exploration and discovery to southeast Asia and the east coast of Africa that have no equivalent in the world for the scale on which they were carried out: a ” imperial cargo carrier” was over 100 meters long and had 8 masts and hundreds of sailors as a crew, for example. Zheng He had several of these ships in his fleet, plus dozens of smaller ones. In total thousands of people went with him. He established diplomatic relations with kingdoms in southern India and the Swahili coast, he intervened in dynastic disputes, cleared the Indonesian straits of pirates as best he could (then as now, those waters were infested with pirates – paradoxically many of them were ethnic Chinese). Exploring, he reached the northeastern coast of present-day South Africa, as novelties apart from ambassadors (the first blacks ever seen in China) he also brought giraffes (they caused a deep impression) and both African and Indonesian rhinoceroses… but after the death of the emperor who was his protector, his son and his nephew (who remained on the throne) refused to continue financing these expeditions because “they are very expensive and there is nothing we need in the rest of the world”. And not only that, there was the ban on building ships with more than 2 masts (so that no one would stray too far from the coast) and then soon the ban on leaving the country. And so, suddenly, China abandoned the place it could have, leading the age of discovery and colonization almost a century before the Europeans. Instead they closed in on themselves… which made the Manchu conquest possible.

If any of these 3 events had not happened, it is very likely that today’s technological level would have been reached centuries earlier… except that the world would be deeply Chineseized instead of Europeanized. THOSE are the real tragedies to mourn in my opinion, more than any of the burnings of the Alexandrian library (which by the way, we should also mourn then the burning of Persepolis by Alexander the Great, in which his library also burned, and the burning of Baghdad by the Mongols, library included, as they contained a significant summary of human knowledge of that place and time)