It generally means "I (as the owner of a coal or nuclear plant) don't want to reduce my output on a short timescale as that has costs for me, so I'd rather pay

It generally means "I (as the owner of a coal or nuclear plant) don't want to reduce my output on a short timescale as that has costs for me, so I'd rather pay someone to take the energy". They mention the hydro dams, but I'm assuming they all have spillways for shedding water without generating electricity, which they would use if forced to pay to generate. Since we already have a grid, it doesn't make monetary sense for every such onflexible producer to invest in their own flexibility unless they can do it for cheaper than the market rate of paying what is effectively a fine every so often.