I doubt it"s the "same" engine really. Engines go through iterations, I imagine porting Horizon to PC is not an easy task by any stretch.Plus HZD and Death Stranding are using the same engine and DS is coming to PC.
It is maybe cheaper to use the knowledge from porting DS to PC, maybe even reuse some tools or code, than to start from scratch with a bigger title.
And if technical feasability was paramount then Days Gone would be relatively straightforward since it's UE4.
I think there is something bigger at play here. Sony must have a launcher of sort in the pipes.