I don’t really get this argument. If somone with enough knowledge about sights, ammo, and long range shooting experience to hit a target with a hunting rifle could also hit it with an AR15. The range in this instance was not excessive and well within the capabilities of both rifles. The difference is an AR15 can put a whole lot more shots down range, much faster. Obviously, I am aware that rapid semi auto fire or semi auto rifles in general are less accurate.
Being able to shoot more rounds can make up for lack of skill. An amateur with an AR15 is much more likely to hit their target or inflict more damage than an amateur with a hunting rifle. Here a skilled shooter could have done it with either.
I have been trying to find this info and have not been able to yet. Short of counting the shots from a video I am not sure we will 100% know until the investigation is done. I will edit my previous comment to reflect uncertainty.
Edit: This article claims the counter sniper who killed him only fired one shot. Although perhaps more SS officers fired?
https://www.cbsnews.com/news/timeline-trump-rally-shooting/