The specific reason he contends it may be an alien artifact is that he believes its characteristics match the profile of a light sail: 1) its shape may actuall

The specific reason he contends it may be an alien artifact is that he believes its characteristics match the profile of a light sail: 1) its shape may actually be more like a pancake than a cigar, a crumpled light sail would match this 2) it showed unexpected acceleration away from the Sun without outgassing. 3) its velocity is very close to the mean motion of our local stellar neighborhood, if you needed to drop a solar sail after deceleration so that it would be hard to work out what system you came from then this would be the velocity to do it at.