That kind of has a multi-part answer.
Minecraft world manipulation must happen on a single thread. Sponge doesn’t change that.
Sponge does however add in things like async lighting, which makes lighting updates happen off thread.
There are a few more things than that, but the general idea is the same. Sponge is able to make certain parts more multi-threaded, but it is not able to change the fundamental single thread-ness of the game.