In the following short of the MIT regarding two blocks connected by a spring, it can be seen that the center of mass of the two-block system moves at constant velocity.
However, in a side question: "What is the speed of the blocks at the instant when the spring is at maximum compression?" the provided answer confuses me: "When the spring is at maximum compression both blocks are momentarily at rest relative to each other but they are still translating to the right with the velocity of the center of mass"
I can see that one block stops momentarily relative to me (as in, I hide with my hand the left block and the spring, and the block moves, and stops, moves, and stops).
How can be said that its velocity is the constant velocity of the center of mass at that point if the block stops momentarily at that instant?