Now, he’s back and ready to return to the old galaxy with his army of unique-looking stormtroopers with him. However, they’re technically not stormtroopers.
The subtitles for the episode reveal that they are called night troopers, and while there have been other iterations of stormtroopers, these ones seem particularly important.
More than a decade has passed since Thrawn was exiled to the other galaxy, leaving him little resources to rebuild his army, which explains why their suits look so ragged.