We already know that fusion power plants are going to operate by igniting plasma in short bursts -- a few seconds, maybe 10s at most -- and generating a huge amount of power during that single burst. You then ignite it again and again.
The question is how much total energy you can extract from each of those bursts. By that metric, the total energy is what's important, not the power. Producing 10 quadrillion watts of power isn't that useful if it's only for 100 trillionths of a second. (Both numbers pulled from an actual recent result last August. [1])
"59 megajoules of sustained fusion energy" means a single burst produced that much. That's significantly more than the paltry 1.3MJ from that other result I just linked to.
Yes, of course you could get that much from a wall socket over the course of hours, but we know we haven't been able to sustain a fusion reaction for a few seconds, so that's the timeframe we're talking about for a single burst.
1. https://www.livescience.com/fusion-experiment-record-breakin...
EDIT: And, at the bottom of the article it says that the reaction ran at 11MW, so plugging that in it sounds like, indeed, it ran for 5 seconds.