Not possible to effect keyframe animations with HDRI Backdrop in level sequencer; not scene component, not geometry (inherited), not skylight (inherited), not via empty actor (pivot).
Further research suggests a previous method using BP_Light studio but is not compatible with specified cubemaps. As stated in the cross-thread, I am astounded that out of the entire scope of functionality available in UE4 (4.26), it is not currently possible to keyframe Source Cubemap Angle, despite the essential function of the cubemap rotation for lighting a scene.
The only solution I have discovered is to literally rotate the entire game world while the skylight remains in place.
+1 the ability to rotate a hdri backdrop around its z axis is only available manually in viewport. Even though rotation on z in viewport works the sequencer transform equivalent for this actor ‘yaw’ has no affect.
such basic functionality would be good to have