Let me make it a bit clearer.
Isaiah 59:20. 'He shall come as redeemer to Zion,
To those in Jacob who turn back from sin
- declares the LORD.'
So, even to a simpleton like me, this passage is clear enough. There will be some in the nation, Jacob, who will turn back from sin, and others who will not.
Now, let's add another scripture. This time from Jeremiah 31:33,34.
'But such is the covenant I will make with the house of Israel after these days - declares the LORD: I will put My Teaching into their inmost being and inscribe it upon their hearts. Then I will be their God, and they will be my people. No longer will they need to teach one another and say to one another, "Heed the LORD"; for all of them, from the least of them to the greatest, shall heed Me - declares the LORD.
For I will forgive their iniquities,
And remember their sins no more.'
Jeremiah tells us that all Israel will be saved.
Now, a little application of logic. If all Israel is saved, but all Jacob is not saved, can the two names refer to all the same people? My thinking says that the two are different.
Can we agree on this distinction?