The inertia issue is well known among power engineers, and the situation described here was entirely predictable given the heavy penetration of “renewable’ energy into the grid.
Nuclear Power Plants can not react in that kind of time frame. Nukes are great base load plants but are poor at quick response to load changes.
Really any turbine plant can not not increase its load by 1500 MW in 1.2 seconds. The torque on the turbine shaft would snap the shaft like a twig and the thermal expansion of the copper windings would likely lockup the rotor.
Back in the good old days of 'Regulated Electric Utilities' the electric utilities kept 'Spinning Reserve' plants running at very low loads for such events. They could quickly ramp up for such system upsets. They also provided the 'grid inertia' the article speaks of.
In the utility I worked for when I started these plants were older, dirty small coal fired plants. In my later years the coal plants were retired and the utility built gas turbine plants that had quicker response times and could be remotely brought on-line.
This all goes to the point that for the grid operator to be able to respond to a grid upset in such a short time frame the grid would have to several plants on hot standby ready to be spun up. No single plant could increase power output by 1500 MW in 1.5 seconds.