My take is that he's arguing that the urge/impulse for political unity within the Islamic world, and by extension the automatic identification with pan-Islamic causes felt by many Muslims, represents an attempt to leave behind the mundane, everyday politics of the modern state in favour of the pursuit of what is a mirage of unity, BUT that in fact the only type of political unity that can be achieved within Islam is the coercive but superficial type represented in this article by the Saudis overseeing of the haj. The conclusion is that Muslims would be better off celebrating their differences and getting on with contributing to politics within the political framework they find themselves in. As he says, "The nation-state is inescapable when it comes to matters of establishing and governing matters within and between religious communities."
I think he's come to this conclusion because despite the urge for political unity represented by the movement to restore the caliphate, no group (apart from a couple of exceptions which prove the rule) has articulated a realistic political proposal to make it happen, or outlined how it will work. The exceptions are of course the Hizb ut-Tahrir, whose manifesto is not only illiberal (even fascist) but farcically amateur, and ISIS about whom we need say no more. Beyond these two, there are a range of groups who claim to have to restoration of the caliphate as a central principle, but shy away from drawing any immediate political conclusions from it.
Finally, he argues that in fact, Islam was at its strongest when it accepted political and theological diversity - "the internal schisms and conflicts of Muslim societies demonstrated a sense of confidence and comfort with disagreement as a political necessity". This is something I've argued here -
https://www.redcafe.net/threads/attempted-coup-dé·tat-in-turkey.371970/page-28#post-19515026
When you speak of 'unity', do you mean political, theological, or what? And if you mean political untiy, why do you think that would necessarily help lead to the type of reform you want to see?