It is true that the maximum speed of a block in a trajectory occurs at its maximum height, where the vertical component of the velocity is zero. This is due to the fact that at that point, the kinetic energy is minimized and the acceleration is zero. At this point, the block is at its highest point, and the velocity of the block is at its lowest. Therefore, the speed is zero at the top of the trajectory.
However, the horizontal component of the velocity is not zero at the maximum height. This is because the block is constantly subject to horizontal acceleration by gravity, even at its highest point. This means that the block has a nonzero velocity in the horizontal direction, even if it is at its maximum height. Therefore, the block is not at rest at the highest point; it is just moving horizontally at a constant speed.