Isaiah's Greek text repeatedly uses ego eimi in divine self-identification passages.
Isaiah 41:4; Isaiah 43:10; Isaiah 46:4 provide significant background.
The saying joins heavenly origin, salvation from sin, and an absolute 'I am' formulation.
No single grammatical feature should be overstated, but within John's cumulative pattern this is strong evidence of divine identity.
Jesus' hearers ask, 'Who are you?'