...I had the EXACT same situation with the 15" Evo I purchased. I ended up using wooden wedges between the tabs for the tighenting screws to open the rail up so it line up properly could slip on over the thermal bushings. I put the thermal bushings on the barrel nut I was going to use in the build and inserted it in the handguard and left it there to allow the handgurd to "stretch" and conform to the bushings while I awaited the rest of the parts to arrive. Eventually, all the parts arrived and I assembled the rifle and encountered lesser problems when it cam to the handguard installation. The HG never "slipped" right on like the video shows, but they did go on without the use of any hammering or such.
I'd check if the barrel is centered inside the tube once you have tightened it all down, it would be readily evident if your front sight requires an inordinate amount of turns up or down (more likely down) to get it aligned to put rounds on target. I'd also check the left to right centering of the barrel as well, again evidenced by inordinate adjustment of the rear sights windage adjustment.
"Samson Pro" is very proactive responding on this forum to posted issues, and advises contacting Samson staff so they can assist or return your calls or make contact with you. My experience in that regard is different from the perception those forum responses give to the viewing public.
Good luck with your install of the Samson product you purchased.