At first I thought your math was wrong, because that doesn't match the numbers in the article. But then I checked the day-ahead prices at epexspot[1], and it's indeed showing -500 euro/MWh in the afternoon, from 14h00 to midnight!

[1] https://www.epexspot.com/en/market-data?market_area=FI&tradi...

They placed a bit at that price doesn't mean the market price will be that high.

It is an auction and everyone pays the same price. During the afternoon tomorrow every household in Finland will be paid significantly for their power usage unless they have a fixed price agreement, which is not that common

According to Finnish Energy Authority (energiavirasto.fi), at the end of 2022 only 13,7% of customers had a contract where pricing is tied to the hourly market rate. It is much more common to have either a fixed price contract (usually for a term of 12 or 24 months) or a contract where the pricing is set by for example monthly averages of the market price.

My mistake! Assumed all Nordic countries were the same

I'd suspect that actually fixed-price agreements are very common, and represent the overwhelming majority of contracts for residents/individuals.

I know that after the war started spot-prices rose sharply and a lot of people made the jump. Some, unfortunately, locking themselves into relatively expensive prices.