Except there is no concept of “individual momentum,” it’s all relative to something. Not to mention, technically speaking, any specific reference point that isn’t the blue portal will actually show it has infinite speed as it instantly moves from one spot to another. I think the most intuitive answer is to imaging standing in front of the blue portal, and look through it. From your perspective, the victims are being hurled at you, propelled by the ground. As soon as they go through the portal, no linger being in contact with the ground, they are effectively projectiles. By no means a hard proof, but this video has a compelling argument for that interpretation.