So, the cube is not moving on the server side, because we told it not to. The server is the entity that has the “correct state” and will not care if the client is doing weird stuff (like moving a cube). Hence, we (client) will still get stuck at the static cube as seen on the server side, while we can walk through it on the client side, since the physics of blocking etc is done on the server side.
The client is also allowed to move the cube since the server does not change its position. A fun experiment is to move the cube, say, once every 3 seconds on the server, in an opposite direction. This makes the cube on the client side glitch and jump to the position where the server believes the cube to be, it will then start moving again, but only on the client side, until the next time the server moves it