Actually hold on, retract my previous statement. By the time of Naga Sadow, there were modern lightsabers around to be used. So the Lost Tribe of the Sith having them, is fine. The Jedi used protosabers yes, but the Sith had the more modern lightsaber before they did.

Hmm I will say that in crosscurrent, the Jedi Master was mentioned as having a powercord on his, and I think the same with the Sith, I'm not sure. These were also miners so not top of the line, I remember an interview where it was stated that the main reason why these sith, unlike the dark jedi that encountered the sith species, couldn't mate with the Keshiri was because they were lacking in knowledge of Sith magic due to being very low level. Plus I saw a picture of Naga Sadow dueling, and he still had a powercord on his. However, I'll also admit my familiarity with aspects of this era isn't the best. As for advancing on their own, it really did look like they even regressed on basic lighting, heck they were using animals for transport still and couldn't figure out even cars. That's just my impression, I'd love to continue hearing yours.

Edit: Alright I see what you were saying, I missed the bit were they developed them first. Alright looks like my research missed something. Though I'm a little surprised they gave their best stuff to what are essentially miners.