decima was sony's though — xbox port means new engine, that's the real delay

No, Guerrilla Games. The engine also already runs on virtually every platform, including the Xbox and PC, but also phones.

Even though Guerilla Games is currently owned by Playstation Studios, they are still effectively an independent studio.

Microsoft, otoh, owns like half a dozen engines, uses none of them, and licenses none of them. The only way I'd see a engine swap happening is if Kojima somehow swung a deal to license the newest version of the Doom engine.