There are several energy markets, not just one. The most energy is bought and sold on private contracts on very long term. Then there is the "next-day" market, which is what this app shows. Then there is an hourly market to match instant demand and supply variations.

It is possible Czech producers sold energy to Germany for a very low price a few years ago on a let's say 5 year contract which is still valid. Now Czech Republic needs to buy at a high price the energy from Germany because it cannot meet its own demand. Again, this is speculation on how that could work.