That wasn't Traviss. She was just following the original Marvel Comics that had Mandalore being low tech.
And that's still canon because the EU decided that maybe the stormtroopers aren't all clonetroopers because that makes handling the canon a lot easier as no pre-AOTC works suggested they were an army of clones.
That is not true actually. The idea of them being clones was suggested in 1981 in the Star Wars Compendium 2.