So the lightsaber is actually old technology? I am confused
Judging from how unstable the lightsaber beam looks - the crystal inside the lightsaber is hand made and imperfect, as if it was homemade. All Sith crystals are created through a process and are manufactured unlike the Jedi crystals - this Sithlord might have tried to replicate the crystal creation process by hand. Unstable crystals cause more damage however can short out randomly - which could be dangerous in a duel.
The design of the hilt itself looks similar to how some of the Sithlord's hilts look in Knights of the Old Republic and The Old Republic. I know that Disney said anything in the EU is non-canon - however from what I read it only counts for anything that's set after Episode VI. The Old Republic Era takes place 3000 years before Obi-Wan was even born, so any events taking place then shouldn't have any effects on what happens in this film.
The "Old Technology" part is referring to how this Sith lord might be thousands of years old, awakened and is now storming around the galaxy causing havoc and their lightsaber reflects this - the lightsaber could be over thousands of years old.