God was not just talking to Ahaz, He was speaking even to US this day. Christ was to become that one and for all time perfect sacrifice so that the judgment pronounced upon the first rebel would be fulfilled, and... no longer was the shedding of blood required for the remission of sin. Isaiah 14:12---- and Ezekiel 28:12---- That is the point of this 'flesh' age so each individual soul/spirit can make up their own mind which path to take the left or the right. God wants all his children to love Him of their own free will. Pure love cannot be forced or bought.
No,G-d was talking to Ahaz. It has nothing to do with j-sus. Ahaz was in dire straits and needed G-d’s help right away. j-sus didn’t come around until 700 years later.