The materials used in AR's are not plastic and hold up in the real world much better than wood. Wood on an AR is done as art work.
The Great M1 Grand and M14 A1 had issues with the wood not holding and replace it was a big business
Big reason they moved away from wood.
A polymer is a plastic. A durable plastic but plastic none the less...
The stocks in the link are a wood laminate probably birch, possibly maple,The laminating process makes for a wood stock that is more durable, stable and also heavier than the walnut stocks common to the Garand and the M14.
If I was going to add an extra pound to an AR-15 it would be a bull barrel or scope, a wood stock is taking it in the opposite direction of its design. They originally wanted a rifle to weigh under 6 lbs. but the early pencil barrels showed some weaknesses.