Why does Absol raid boss still have the same CP now as pre-rework?

Because its defense didn't change in the rebalance and base HP does not affect raid boss CP, since its normalized to the raid tier.

Main series has the same defense and special defense, so when the CP rework happened, its defense in GO didnt change. and raid bosses have fixed HP that didnt change.

Theres a few pokemon that are raid bosses that this rework wont affect the CP of, Rayquaza, Giratina (both forms) Deoxys (I think all forms) Absol, Registeel, had cresselia been out before the rebalance Cresselia, and probably a lot more.