The verse is part of the final section of the haftarah for Yom Kippur afternoon and also figures in the siddur in U-va L’Tziyyon.
It is possible that it is to be read as poetry and simply means that God will not abandon His children. In this sense “Jacob” and “Abraham” are metaphors for “The people of Israel”.
But if this is the case, why is Isaac left out? And why is Jacob linked with truth and Abraham with lovingkindness?
An answer to the first question: Jacob is the father of the community of Israel, since the tribes are named after his children, and Abraham is the founder of the faith who discovered God and taught belief in Him wherever he went.
The second question is answered by Samson Raphael Hirsch with a reminder that both patriarchs feared the future (re Abraham: Gen. 15:1; Jacob: Gen. 28) and both needed Divine assurance that God would fulfil His promise to grant them a future (this is why Micah says, “…as You promised our ancestors in days of old” (Micah 6:20)).
Another possibility is that Abraham needed a special lesson in lovingkindness because there were times when he did not plead hard enough for others (he did not press God to save Sodom and Gomorrah even for the sake of one righteous person), and Jacob needed a lesson in truth because there were times when he allowed himself to deceive others, in particular his father Isaac.