What is the significance of the Negus's statement: "Indeed, this and what 'Isa brought forth both emanate from the same niche"?

The intent behind Negus's statement: "They emanate from a single niche" is that the Quran and the Gospel share a common origin. The 'niche' (mishkat) refers to a non-penetrating recess or the iron hook from which a lamp is hung. He meant that both the Quran and the Gospel are the word of God and originate from one source. Negus was on the true Christianity, so he affirmed the truth he heard in the Quran and subsequently embraced Islam.