If I use Hoopa's "Assault Gate" attack after I move it up to replace a Pokemon that was KO'd by an opponent's attack, does it do anything?
No, Hoopa was moved to the Active Spot during the opponent's turn, not during "this turn".
Source: Darkness Ablaze FAQ (2020-07-31), TPCi Rules Team (2020-08-13)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH3_111
SWSH3_111