Then Abijah stood up on Mount Zemaraim that is in the hill country of Ephraim and said, “Hear me, O Jeroboam and all Israel! Ought you not to know that the LORD God of Israel gave the kingship over Israel forever to David and his sons by a covenant of salt?
(2 Chronicles 13:4-5 ESV)
This is the only mention of a "covenant of salt" with respect to David. How is Jeroboam to know the LORD God gave the kingship over Israel to David and his sons, by a covenant of salt? The other reference to a covenant of salt is in Numbers 18 where the LORD God makes a covenant of salt with Aaron:
All the holy contributions that the people of Israel present to the LORD I give to you, and to your sons and daughters with you, as a perpetual due. It is a covenant of salt forever before the LORD for you and for your offspring with you.” (Numbers 18:19 ESV)
When David brought the Ark of the Covenant to Jerusalem, he first built a portable Tabernacle like Moses did, then he acted as Aaron the high priest would:
And they brought in the ark of the LORD and set it in its place, inside the tent that David had pitched for it. And David offered burnt offerings and peace offerings before the LORD. And when David had finished offering the burnt offerings and the peace offerings, he blessed the people in the name of the LORD of hosts. (2 Samuel 6:17-18 ESV)
David offered burnt offerings, peace offerings, then blessed the people as Aaron is instructed (Numbers 6). In summarizing David's leadership over Israel, his sons are called priests:
and Benaiah the son of Jehoiada was over the Cherethites and the Pelethites, and David's sons were priests. (2 Samuuel 8:18 ESV)
In particular, Chronicles describes the extent to which Solomon carried out priestly duties:
Then Solomon offered up burnt offerings to the LORD on the altar of the LORD that he had built before the vestibule. as the duty of each day required, offering according to the commandment of Moses for the Sabbaths, the new moons, and the three annual feasts—the Feast of Unleavened Bread, the Feast of Weeks, and the Feast of Booths. (2 Chronicles 8:12-13 ESV)
The "revised" history in Chronicles describes Solomon functioning as priest for at least one complete annual cycle.
Is Abijah using a covenant of salt to say David and his sons are rightful priests as well as the rightful king?