That's different. The nuclear power plants absolutely respond to the grid operators (and always have), but it's the reactor operators moving the power level rather than the turbine and generator automatically adjusting to load changes on the grid. Big difference! In my experience this situation was fairly rare, but I didn't operate in a de-regulated market like most of Exelon plants.
From your citation: "The site receives the signal on a computer in
the Main Control Room, operators confirm the dispatch signal with the Constellation Generation
Dispatcher, and then a Nuclear Regulatory Commission-licensed Senior Reactor Operator at the
site authorizes the load reduction. Reduced power is maintained until a dispatch signal to raise
load is received."