Development of HellDivers 2 started in 2016. So they would have been 2 years into development with that engine. So they would have had to effectively start again in another engine.

Wow, that is a very long development cycle. It really shows in the level of polish this game has.

That's really strange. Perhaps they had a set of engineers that knew it very well or had built an engine in it for another game. Still... strange.