The original comment was about lithium and their popularity for backup power. Sodium ion batteries are so new that you can't purchase them yet (blueitte supposedly released the NA300 but I can't find any in stock and it's no longer on their site).

It wouldn't be fair to compare a chemistry you cannot purchase and which it's strengths and weaknesses haven't been tested outside of controlled laboratory testing.

Probably because they're new and the parent comment specifically referred to the cheaper, less energy dense battery types.