The Heretic Astartes codex Daemon Prince entry DOES replace the Index Daemon Prince entry. Though if the wargear options were different, you could still select index wargear if you wanted. But I think the options are the same.
You could take a Nurgle Faction detachment with a Nurgle Daemon Prince with Warp Time (probably Alpha Legion). You could also just bring an allied CSM detachment with a warp time jump pack sorcrer of Nurgle, or just a CSM detachment and then pay 1 CP to replace any power (including Smite) on a Death Guard psyker with Warp Time.
Nothing is overriding the option to take a Daemon Prince from the Index Death Guard list though. Unless Daemon Princes lose their access to warptime in the codex version, you can still take a Death Guard keyworded Daemon Prince with warptime
That's how I play it and it is correct. FWIW Battlescribe also allows this.