In the oldold days there were aluminum handguards like that which were one piece, the barrel nut was integral with the rest of the handguard. I do not know if the one on that rifle is like that or not. If that's what it is, the only way to remove it is to remove the gas block and gas tube, put the upper in a vise block and twist off the handguard with a strap wrench.
Hopefully someone will be along that is familiar with that exact rifle to know how that handguard is made.
ETA:
That's what I figured from the looks of it.
I had a similar one on an old rifle and broke the strap wrench I bought trying to remove it. Ended up making my own strap wrench from a piece of 2x4 and some 1 inch heavy nylon webbing. The 2x4 was about 2 feet long to get enough leverage. Once I got a strong enough strap and enough leverage it came off without additional drama.
Actually come to think of it I took one off another time, using a ratchet strap as a strap wrench. The ratchet handle became the torque handle once it was tight. It actually worked really well. I put a little piece of heavy
rubber leather in between the handguard and the metal to prevent scratches.