It is impossible to change the general momentum of an object in micro gravity without an outer force that alters the momentum. There can be no change of momentum out of thin air. Remember Newtons 3rd law. The object can not do this by itself. The video of the guy extending and contracting his arms is simply him changing 'the length' (or radius) of his body which influences the rotational speed, but not the momentum in any way.
You're right. The general momentum or force, or even "spin" doesn't change in the video, however, if you think of yaw, pitch, roll, you can see that they're making spins around their axis and even tumble, without losing the momentum or general spin they have, so you can alter certain spins that you have. Since the post I responded to was mentioning "spin" and not "momentum", it wasn't a matter of the total/general force etc, but rather the local "expression" (if we can use that term) of the momentum.
Put it this way, extending arms and pulling in to change your spin doesn't change the momentum (scientific term)... but it still does change your spin (non-technical and non-scientific term).