This is a real problem but you can only have so many words in a tweet. Note that the price isn't zero but instead negative. It means there is literally too much power in the grid and it would need to be used. If a grid has too much power then it is bad. It can damage it. There are things we can build that essentially amount to batteries (or natural variants like a dam) that get charged during times of higher supply than demand and discharged during times of higher demand than supply.