John Gill Commentary 2 Chronicles 13:6

"Yet Jeroboam the son of Nebat, the servant of Solomon the son of David, rose up, and rebelled against his lord." — 2 Chronicles 13:6 (ASV)

Yet Jeroboam the son of Nebat, the servant of Solomon the son
of David, has risen up
Notwithstanding it was well known the kingdom was entailed on the posterity of David by an everlasting covenant; Abijah calls Jeroboam Solomon's servant, by way of great contempt, as Jarchi observes, he being the general receiver of his tax in the tribe of Ephraim, (1 Kings 11:28)

and has rebelled against his lord ;
his rightful king and sovereign; the charge is no less than high treason.