The declaration of Khilafah by ISIS is, of course, a statement more of ambition than of reality. Obviously, Abu Bakr al-Baghdadi has not assumed control over the affairs of the entire Muslim Ummah, nor is there any extent mechanism by which he or anyone else could do so.
Rather, the Muslims are being invited to achieve the Khilafah themselves essentially by agreeing to rally around al-Baghdadi and commit to obey him in whatever he says. As if the only thing preventing the re-establishment of the Khilafah was the absence of anyone willing to stand up and offer himself as an absolute ruler.
In order to actually achieve the all-inclusive Khilafah ISIS imagines, obviously, the Muslims have to go along with it. It is a bit like saying, “I could let everyone live in my giant castle, if everyone would just come and build it for me.”