Your description is right on. The stuff is molded, since the details would fuzz off if it was routed. It's not unlike a low grade of tempered hardboard. I've seen this kind of stuff used for exterior lap siding in NC, and it was the subject of a class action suit due to it taking on water, swell and soften back to its cardboard heritage. When broken, it doesn't break cleanly but in layers. It has pretty decent impact strength, since it is almost impossible to dent with a hammer. . . though you'd like to. The stuff is found even in multimillion dollar McMansions here in the southwest, and I'm sure it's coming to a Home Depot near the rest of you. When you cut it on the edge, as around a door, it'll cut and hold the 45. However, on a baseboard, cut on the flat, the joint fuzzes and looks like crud. Coping the joint is impossible. That and the fact that most of the trim folk in the southwest don't even own "no steenkin" miter saw, is why the baseboards are cut square and fitted to a transition block on outside corners. I'm kidding here, of course, since the transition is needed because of the 1" radius used on almost all outside drywall corners.
BTW, getting back to the cardboard trim, it isn't very water resistant. I was in a mountain cabin once and noticed a very unusual look to the door trim of the guy's back door. The bottom of the trim was swelled out, both from the wall and parallel to the baseboard, up about 6" from the floor. The door had some water that came in by the threshold and the water was wicked up the unsealed bottom of the cardboard trim, swelling it like a sponge.
I'm all for progress in housing materials, but most of the "new stuff" of the 90's and 2000's is a step backward.