A sophisticated likely state-sponsored spook could have a compact heads-up-display weapon for assassination, agreed.
However, it would almost certainly be a smaller caliber or a flechette-style projectile. But also, I would almost expect a .30 to make it through the neck at the relatively-short range from the roof. Though there were claims that his spine got trauma, but that could be from cavitation or even just shock to the brain and nerves concentrated that close to the impact.
I could see 5.56 or 5.45 not making it through the neck, and that was what I assumed this was going to be before they announced it.
5.45 in particular was known as the poison bullet because of it's tendency to enter the body and spall to damage internal organs, but 5.56 is similar because of it's speed and relatively low mass. Also why it's not the best hunting round in forests I would assume.

But to me, I don't know if the video is real or fake. I just don't see an obvious execution there from that guy.
There's also the fact the state actor would be conscious of the sheer amount of cameras in multiple directions there near Charlie.
There was an above-the-shoulder camera mounted at the back of that tent that was removed 5 mins after the shooting, and before police secured the scene.
That camera likely has footage that would prove more regarding this guy as I'm sure he's in frame.
To go through all the effort of having that guy there to do it, and then be caught on camera seems counter-productive UNLESS you had guaranteed cooperation from local authorities/federal in Utah.

The text messages from the alleged shooter are also quite strange, but they could be written in an obtuse way in an attempt to clear a network of other people involved with Robinson.
Long before this there have been lots of murders involving trans individuals associated with online networks such as 09A:Order Of Nine Angles.
Also he could be trying to clear his "lover".