It's recording power over time. From the article this is since April, but without that to bound it doesn't really give the relative scale to compare it. It could be lower power over a long time or a lot of power over a short time.

I'm also not really sure that's impressive. It sounds like a lot because "billion" is big, but that's around the amount of renewable energy my metro area uses in a year. Total use is around 10B.

Its also a really small unit of energy for a power line, it's like "my car went a billion inches!"