If your textbook indeed says this, it is evidently biased. First of all, there things (the Caliphate, the Mongol Empire, and European empires) belong to very different historical periods, and thus cannot be compared. The "world standards" of what is considered "benevolent" and "tolerant" are changing with time.
For example, in antiquity and during most of the Middle age slaughtering the whole population (including animals!) of a city after a successful siege was considered normal. Slavery was considered normal. And the idea that all humans have some rights is very modern (18s century).
Second, each of these entities had a long period of evolution. Take the Mongol Empire, for example. During the time of its creation (Mongol's conquest) it was extremely cruel. This was one of the greatest disasters in the recorded history in terms of the number of people killed. As some point there were intentions to exterminate the whole population of China, and turn it to pasture. This was prevented by a (Chinese) adviser of the Great Khan, who proposed a plan to tax the population instead, and counted the benefits. But later Mongols ruling China became assimilated, and in the later generations they were not very different from other Chinese rulers.
Similarly, AFTER the conquest, and after establishing Islam firmly, SOME Muslim rulers were relatively tolerant, in comparison with their CONTEMPORARY European rulers. But comparison of the early Muslim rulers with the much later European empires simply makes no sense. Like comparison of the modern "Caliphate" with
the early Caliphate.
As an example of Muslim tolerance they usually cite that ancient Greek and Roman learning was preserved, and even developed, at the time when it almost disappeared in Europe. But it is rarely mentioned that the whole Persian pre-Islamic literature was lost. It just does not exist. And Persia
was a highly developed culture at the time of ancient Greece and later.
Also the earliest European overseas empire (the Portuguese one, 16s century) was very different from the 19s century British empire.