Yes Isaiah is using figurative speech. You are projecting a later teaching onto Isaiah.
The Holy Spirit says otherwise. But the language was plain enough.
Learn this lesson: never declare a direct statement of fact as 'figurative' unless there is very good reason to do so.
Isaiah was speaking of those who were living in spiritual death and Christ came to give them life. Nothing could be plainer than that. Not only so but Matthew recorded the fact that Jesus fulfilled what Isaiah said he would do...literally.
"That it might be fulfilled which was spoken by Esaias the prophet, saying,
15 The land of Zabulon, and the land of Nephthalim, by the way of the sea, beyond Jordan, Galilee of the Gentiles;
16 The people which sat in darkness saw great light; and to them which sat in the region and shadow of death light is sprung up.."
Now, Zabulon, the land of Nephthalim, Jordan, and Galilee ARE ALL real, literal places. Jesus, being the Light of the world came among those who lived in those places and had no light and brought them life; life from death.
You are picking and choosing what you will and won't accept as literal/historical based on your prejudices, not on fact.
Without realizing it, your insistence upon the text being 'figurative' renders it useless. If the language of "The people that walked in darkness have seen a great light: they that dwell in the land of the shadow of death" is not darkness and death in substance (reality) then what, pray tell does it mean?
Your interpretation is in error. God told the truth. On the day that Adam sinned in the garden of Eden he died. First his spirit died and his body began to die. God does not lie. 'Let God be true and every man a liar.'
"Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life." Jesus in John 5:24.
The Lord Jesus Christ, being co-Creator with the Father did not lie about this and neither did His word lie about Adam and his sin. That sin brought him death on the day he committed it. The death God spoke of concerning Adam is the kind of death Jesus mentioned above.
This discussion is looking at the simple reading of the text, not hidden meanings.
I didn't say anything about hidden meanings as it concerns Proverbs. Proverbs is neither a tale nor a story. It is a philosophy of life and the Lord expects His people to live by those proverbs that Solomon wrote because He inspired him to write them.
You aren't telling the truth.