They don’t mention the actual specific energy of the completed cells in Wh/kg. Also, “four times of WHAT?” They need to compare like-to-like, which would mean comparing to an equivalent Li-S cells.
Li-S cells usually have much higher specific energy than regular lithium ion. I doubt these cells are better, considering sodium is heavier than lithium.
The highest lithium ion cells you can get now are Amperium cells at 390Wh/kg, plus the metal anode Licerion cells at over 400Wh/kg. That’s not counting lithium sulfur which can get to 650Wh/kg (but are still stuck in the lab).