It's an option, too, but Tris claim/lynch in this situation is more favorable. Let's say Tris flips town. Then he is lying for sure, either he is Exe or not, Vig or Jailor should kill him tonight.
If Tris is scum, then Lanji is also lying and town killers should claim his head.
Like Flower said, he wouldn't be putting himself in a situation wherein he would be viewed as scum.