With a tape measure, i checked whether the length the rifle probably was could likely be concealed. Barrel down the pants to about knee level would still allow some mobility while the stock end of the rifle could be concealed under a t-shirt. The sling would have to have been adjusted for it to hang right. The stock wouldn’t have to be under the t-shirt. I can’t see the video well enough to see if it was done that way, but i think it would be possible.
That still leaves the question of why he would take it with him if he was going to abandon it later. Maybe thought he could get away with the rifle but then decided he better not risk it?
I do not think the guy was a pro, just a decent shot with a rifle.
If Cops roll up, he’s using his weapon. He abandoned it after he was alone in the trees and had escaped, in his mind.