Heburntthebonesofthepriests - כומריא kumeraiya, the kemarim, says the Targum. See this word explained, 2 Kings 23:5; (note).
SDA Bible Commentary (limited)
Volume 1-7
5. Of the priests. Not necessarily of all the idolatrous priests; for many seem to have been left till Josiah’s 18th year, when the king moved the priests of Judah to Jerusalem and slew the priests of Israel (2 Kings 23:5, 8, 9, 19, 20). The latter were not Levites (2 Chron. 11:14, 15).