وَهُوَ أَيِ اللَّهُ تَعَالَى مُتَكَلِّمٌ بِكَلَامٍ هُوَ صِفَةٌ لَهُ
God is a speaker by speech that is His attribute.
Mutakallim is a derived active designation. Predicating such a description normally affirms its relevant underlying meaning of the subject. Here divine speech is not reduced merely to the creation of sounds in another creature.
Objection: would creating speech in something else suffice to make God a speaker? The reply distinguishes an attribute subsisting in a subject from an effect produced elsewhere.