Probably. That or they used the material preview, or they had visualizers that weren’t reliant on sampling the gbuffer data. Unreal has been deferred for over a decade though, since the earliest versions of UE4 when I first started using it. Can only speculate what it was like before that.
Old game dev tools weren’t exactly feature rich though so it wouldn’t surprise me at all to find out they had no good way of visualizing the scene normals back then.