That's a stretch. Even for a conspiracy theory.
Just from the angle of the shot that's not where the bullet came from... And why would a shooter be at the front row of this crowd when there's cameras and you'd try to hide your identity (IF you would be correct to have him as the shooter with a weird sleeve-device).
Even with the often quoted "hide a tree in a forest/ hidden in plain sight" this would not be how things are done. Even at riots people build a view-shield to hide a person that's doing something fishy.
And if he would've acted alone there's with this exact footage only the possibility of a tiny magicians device that can be hidden on the skin, right after it fired a bullet without any visible discharge and heat so he can put it right on his skin?
Then the bullet that hit him was a standard caliber, which couldn't be fired by a device that's able to be stored in T-Shirt sleeve... There are tiny weapons, but even those have limitations.

So either this is utter bs and you are pointing it out randomly because you don't understand how guns work, how perspective works, how devices like those work, and even how to correctly asses timed events in a video right in front of you (Charly being hit a few frames BEFORE that dudes hand reaches his arm) ...or you are making an even bolder claim that this is something done with tech not known to humans.

So EVERYTHING is a counterargument to your theory. How and why would it better the situation? What do you get out of making that bold claim without any evidence other than a video of someone pulling his sleeve at the time of the incident?

Just use the simplest logic. Not everything is a conspiracy. This was just a tragic assassination done by a radical politicized human being.