I've successfully cast some one off aluminium parts. It's kinda middle of the road as far as amateur casting goes. As was mentioned there's a couple more steps then pouring lead in a mold. Then you have to get around the anodizing issue. Remember the added hardness of the anodizing is actually useful in some parts of the unstressed lower.
The thing is, for something like an 80% AR lower, the machining is pretty dead simple. Dimensions don't really matter all that much. The only really critical parts are the pin holes and even those have functionally pretty loose tolerances. The Polymer 80% lowers are even easier. People routinely successfully "Machine" (hog out by hand) them with a cheap jig and a dremmel. So for something like an AR lower, there's not really all that much to be gained, other than custom colors and a cool story, from the kit in the OP. The other options are really pretty beginner friendly.
I've even seen a functioning AR that was done on a drill press with pieces of steel flat stock stacked together. No machining at all.