Does completing the Hoenn Dex in Pokemon Emerald mean that you need to find the legendary Pokemon?

Within reason. You need to have Raquaza, Groudon, Kyogre, The Regis, and the Latis. All others legendaries (Jirachi & Deoxys) aren't needed. Yes that is right, and also here ae some action replay codes for deoxys in emerald: Use both of these and go to the ferry in lilycove an talk to the ladythat ets you into the ferry, then turn the action replay off. on the island there is deoxys. Also i dont consider this as ceating as there is no other way to get deoxys apart from going to a ninteno event. Now even when your not using the action replay you can just go to the ferry and go to Birth island (where deoxys is, you can only catch him once) Replaces item in PC slot 1 with Aurora Ticket:

667FC137 C4C35C38 Faraway Island & Birth Island Enabler:

C0443E89 7B97FEC2

0976A51E FE4031B6