That is not the case: the magnet pulls the (ferromagnetic) spoke bit / needle with the string through the valve hole, then you can thread an alloy nipple onto it and pull it back inside the rim and out the spoke hole.
So you’ve fed your string through your spoke hole, out the valve hole, then put a nipple on the string, dropped that back into the spoke hole… then what? You now have an unsupported nipple in the spoke hole, how do you attach the spoke? You can’t brace it from above as you normally would with a nipple driver or screw driver or owt. Surely the moment you poke it with a spoke it’ll retreat back into the rim? Do you have to hold it with pliers or something?
Jason’s solution of getting them to build them seems infinitely more sensible, particularly if there’s no extra cost.