Albert Barnes Commentary 2 Chronicles 29:24

"and the priests killed them, and they made a sin-offering with their blood upon the altar, to make atonement for all Israel; for the king commanded [that] the burnt-offering and the sin-offering [should be made] for all Israel." — 2 Chronicles 29:24 (ASV)

All Israel - Hezekiah's goal was to reunite the entire nation of Israel, if not into a single state, then at least into a single religious community.

At the time, the northern kingdom was approaching a state of anarchy, and its end was clearly near. Hoshea, the king contemporary with Hezekiah (2 Kings 18:1), did not rule as an independent monarch but as a vassal of Assyria (2 Kings 17:3).

In light of these circumstances, Hezekiah planned to invite the northern tribes to return to their original spiritual allegiance, even if not to their former political one (2 Chronicles 30:5–10). Therefore, to prepare the way for this reunion, he included "all Israel" in the expiatory sacrifice that preceded his restoration of the traditional worship.