How is Allah a Witness of Deity for Himself?

How is Allah swt. a Witness of Deity for Himself in this blessed verse “Allah is witness there is no god but He, and so are the Angels and men full of knowledge. He is the upholder of justice. There is no god but Him, the Mighty and All-Wise. “ Surah Al-Imran: 18. Note that He is independent of His servants, and He doesn’t need to testify of His Deity being present in the ones who know Him. That is why Al-Hussein Ibn Ali (a.s) said: “Blinded is the eye that doesn’t see you”. Is there another God except for Him, or is there a higher status than Him? Allah is Exalted above what they ascribe to Him