Animats explained the meaning of rejected energy in the diagram below. It's efficiency losses in the entire system (I wasn't aware of this terminology). The actual size of load balancing energy used is quite small.

I didn't know about Seetee but potentially there are some minor opportunities to mine bitcoins (effectively as an on-demand load) to help balance the grid at short time scales. It also makes sense for a Norwegian Energy company to do this. However this is a tiny amount, especially compared to the efficiency losses the author is presenting in the graphic. I looked up some data I had handy right now and I think we are looking at a few percent of the overall energy consumed at most.

Isn't that a waste of the capital used for the Bitcoin mining hardware itself? Why would a miner shut down their profitable operation simply because a coal plant turned on?

The miner and the power plant operator are often the same entity. They turn the mining equipment on when they want to dump energy, and off when they have more productive places to sell it. Seetee is a good example.

If you own the mining hardware, why would you do that? You are better off running that hardware 100% of the time (using batteries or similar when necessary). Mining hardware is expensive.

Only when you are actually better off. isn’t there a crossover point between the outside market needing/paying for electricity and mining? Or is mining the most effective return in all cases?