Context is everything: this excerpt is from an article by Nick Danforth in the 11/19/15 issue of Foreign Affairs, "The Myth of the Caliphate, The Political History of an Idea." You have to read what precedes the excerpt. I summarize: ISIS is enamored of the idea of re-establishing something called "the caliphate," a Muslim state, so called because the ruler of such a state holds the title of caliph. There isn't such a thing today because in 1924 Kemal Ataturk, the founder of the modern state of Turkey, officially abolished the caliphate that had been the Ottoman empire. The empire as a political unit had been destroyed by its choosing the losing side in World War I.
That last caliphate (there had been others) had a good run of over 400 years, having been established by Sultan Selim the Grim in 1517. Selim took control of the Muslim holy cities of Medina and Mecca and declared himself the protector of the hajj, the annual pilgrimage of the faithful to Mecca. This type of unified political and religious power is what animates the nostalgia of ISIS for a caliphate.
Danforth asks us to view the idea of the caliphate with this fact in mind:
To put the title grab [of Caliph] in perspective, when the Ottoman
Sultan Mehmed II conquered the Byzantine capital of Constantinople 64
years before Selim conquered Egypt, he [Mehmed] had claimed the title Caesar of
Rome for his descendants.
Constantinople (now Istanbul) was the capital of the Eastern Roman Empire. It was a bit fanciful for Mehmed to call himself "Caesar of Rome," since the Western Roman Empire (with Rome as its capital) had fallen to the so-called barbarians in 476. But "Caesar" wasn't an entirely ahistorical title.
Now we can make sense of the excerpt:
To the extent that being caliph had any more purchase than being
Caesar for the Ottomans in the late nineteenth century, it was
largely the result of a political campaign on the part of Sultan
Abdulhamid II to rally anticolonial sentiment around the Ottoman state
and to boost his own domestic legitimacy.
This says that perhaps it was better for Abdulhamid II to call himself
Caliph as opposed to using the title of Caesar that his long-ago predecessor had adopted, but Abdulhamid's success as a ruler came from exploiting anticolonial feelings. (During Abdulhamid's rule the Russians had encroached on Ottoman territory in the Balkans; the British, in Egypt and the eastern Mediterranean.)