Just kidding 
So how long till UE5?
SDFGI is in UE5, which should be coming early 2021.Don’t know how much of Lumen will be in the preview though.
DFGI has been in Unreal couple of years ago or so. The quality was so terrible Epic has given up on the idea eventually. You can clearly see in the Godot videos that their SDFGI implementation has the same quality issues which makes it pretty much unusable for majority of production scenarios.
I am 99% sure whatever GI solution is in UE5 is not DFGI. My bet is either denoised brute force ray tracing or denoised final gathering, both of which are already available (in admittedly half-■■■■■ state) in UE4.
“Lumen traces rays against a scene representation consisting of signed distance fields, voxels and height fields.”
Sounds like DFGI to me, with SSGI for small details and voxel/HFGI for large scale details.