>>29890107Correct on all accounts.
Since Mirror Move is Flying type Flyinium Z is needed for activation. Using Z-MM gave it the +2 attack by default while also carrying the move's main property of returning the last attack it was hit by. Since it was hit with Icy Wind, it used Subzero Slammer with the properties of Icy Wind, likely lowering the Gastrodon's speed.
The question still remains on if a normal Mirror Move can copy Z-moves or just the base of the Z-move, but it's most likely the latter.
