Quoted: It bad juju to thread a barrel with a die, you need a lathe, or it wont be straight.

This is most likely true for most people, even those who have purchased a thread alignment kit.  In addition, the outside diameter (O.D.) of the barrel may have to be reduced to a smaller diameter in order for the threading die to fit.  This is also best accomplished on a lathe.  Finally, the Stoll F.S. variants are timed, which is to say that the threads start and end at specific points on the muzzle so that the cut-outs in the F.S. are at 12, 3, 6, and 9 o'clock.  This is to aid in slipping them over wire obstructions.  Belgian combo device also has timed threads.  This becomes obvious when the optional blast diverter/compensator is affixed to same.
timing the threads is also most easily done when using a lathe.