I'm thinking it's probably not a matte line. I don't have the opportunity at the moment to look at the shot in motion, but it looks to me like the dorsal phaser array. Notice how the thick line stops just about where the phaser array would end at such an angle? I think the ship is simply aligned to camera in just the right way to make it look like a matte line. As you correctly point out, why would a CGI shot have that problem?