Well based on what I just read this game is essentially cross gen with a cancelled PS4 version to capitalize on PS5 sales. Oh boy....certain people on the other side talked a lot of s%^* about Xbox cross gen so the idea that this game development started on PS4 makes me smile :)
Think of Kameo and Perfect Dark Zero. Two games built for Xbox, upgraded them for 360 and cancelled the last gen version.
Yeah, cross gen would be terrible, but this will be PS5 exclusive built from the ground up, unlike Xbox Series X exclusives being anchored to the Xbox One.
Why does it matter if development started on the PS4? GG could of had PS5 devkits by 2018, the decima engine being refined for the PS5, pushing the SSD without having to optimise for the PS4, etc, the timeline is not clear. For all we know, a ton of assets for HZD2 could have been created on the PS4, but the development of the game shifted to PS5, which means stuff like level design (SSD), enemy types (IIRC flying was too much for the PS4 hardware), NPC's, quests, world size, were all started on the PS5.