There is a really good interview Larry Hyrb (from Xbox) did with Todd Howard, Peter Hines and Phil Spencer about the this.
Source: https://www.youtube.com/watch?v=WI2IPeocbAA
Interestingly at 16:58 Larry asked Todd about Starfield and he says "once we got on the new systems, the things we found we could do .. and so, its a major engine re-write".
So;
A) he said systems and NOT system - so hopefully this means that its coming to Xbox and PS5
B) Since Bethesda's engine came up in this thread, that the engine is being largely written.
This probably also explains why nothing has been seen of Starfield since that 2018 teaser trailer, it also sounds like its a long way of yet. 2022 anyone?