Islam has no body that can do what an ecumenical council claims to do. There is no office with jurisdiction over all Muslims, no synod that can bind, and no mechanism for making a decision that is binding by virtue of who took it. Authority in Islamic law has always run through scholars individually, through the schools they belong to, and through ijma, the consensus of the community, which is recognised after the fact rather than declared in advance.
The Amman conference did not pretend otherwise. It produced a declaration, not a decree, and its force is the number, the range and the standing of the scholars who signed it. Endorsements were gathered afterwards from several hundred more scholars and from a number of international Islamic bodies, which is precisely how consensus has always been demonstrated in Islamic law: by accumulation and by the visible absence of qualified dissent.
Whether that amounts to ijma in the technical sense is the interesting question and it is one the process cannot settle from the inside. Supporters point to the breadth of the endorsement across Sunni and Shia authorities, which is unusual. The machinery to certify a consensus does not exist, so the claim remains a claim.
For an atlas that covers many traditions, this is a useful case. A body can be council-shaped, deliberate carefully, decide something serious and matter a great deal, without having the kind of authority a Christian council asserts. The absence of that authority is not a defect in it.