It means the corps selling at the 'agreed upon prices' are raking it in. Read further and you'll find:

"But the negative prices signal an imbalance in the market and could discourage future investment in more energy infrastructure. Adjusting consumption patterns, such as when EV drivers charge their cars, could help smooth out prices."

Can't be letting the proles have cheap or free energy now, can we. How else are we supposed to know our place if we're not scratching in the muck desperately trying to make ends meet?