I did not assert that it would be legally binding. However, it is considered to be quite authoritative by lawyers, including military lawyers. The two most controversial parts concern the idea of "continuous combatant function" to define members of an armed group, which some want to see defined more narrowly or more broadly (latter: US), and recommendation IX. However, the criteria for direct participation on hostilities are widely accepted as the authoritative interpretation by States and scholars of that term in the Geneva Conventions.
Of course, the document itself would not make a statement on its authoritative nature since, despite the broad consultation with experts, they cannot predict the wider reaction.