Light Weapons?

Hi,
What the heck are the "light weapons" in this game? Most of the videos I find referencing this achievement say that it's the Vanguard's secondary weapon (Saber or short sword), but I have quite a few kills with both of those and there's no progress toward the achievement (playing on standard servers).
Is it broken?

Please help.

I didn't find the right solution from the Internet.

References
steamcommunity.com/app/219640/discussions/0/305510202675653133/
Sustainable Energy Video

Thanks