It’s possible for two people to have radically different ideas about someone but still to be referring to the same person. Someone who talks to Superman is talking to the same person a co-worker of Clark Kent talks to, though the latter has no notion of Kent’s superhero identity. In the same way, Muslims and Christians have different ideas about God but nonetheless worship the same God.
But this argument is question-begging. The analogy assumes there is one being, the same being, with whom Muslims and Christians are both in contact, even if they don’t realize it. But isn’t that exactly what was supposed to be proved? At most, the Clark Kent/Superman analogy shows there are possible situations in which persons can have radically different ideas about someone with whom they are both in contact. In some cases, radically different ideas may still mean the two persons are referring to the same being, but in other cases they will not. It’s not a trivial thing to conclude two groups are referring to the same entity when there’s plenty of reason otherwise.
Why should we think Christians and Muslims are like the acquaintances of Superman and Clark Kent? After all, Christians presumably don’t believe most Muslims, when praying to Allah, are experiencing real contact with the one true God. And Christians definitely shouldn’t believe Mohammad had a genuine encounter with God that led him to found Islam. The Clark Kent/Superman analogy, then, doesn’t contain any special insight requiring us to believe Muslims and Christians worship the same God.