If you scroll down a bit further you might find Brittanica's better definition: "A writing system, technically referred to as a script or an orthography, consis

If you scroll down a bit further you might find Brittanica's better definition: "A writing system, technically referred to as a script or an orthography, consists of a set of visible marks, forms, or structures called characters or graphs that are related to some structure in the linguistic system." Emoji certainly classify as related to a "linguistic system" even if you don't think they have a direct verbal counterpart. Non-verbal languages certainly exist and many of them have forms of writing systems and some of those are encoded in Unicode, too. Emoji isn't the only "writing system for a non-verbal language system" encoded in Unicode.