The thing isn't opaque and glowing all 'round like a fluourescent tube
No, but as you mentioned, it's made of water vapor that disperses and reflects light, even light coming from behind, which allows it to appear almost internally lit when seen against a darker sky.
Check out the edges of the cloud in this pic. Its edges are bright because they're scattering the sunlight that's coming from behind:
Now look at these stills from the video. Doesn't the contrail look like it could be similarly backlit? It even shares the characteristic of being darker in the middle where it's thickest:
The faint shadow it casts in the sky is also consistent with it being lit from behind.
But I don't think the lighting argument is decisive because the light is coming in at such a grazing angle that it's hard to judge exactly what it's doing.
To me the Cargo Law webcam pic is the clincher.