This is wrong. Nuclear reaction output can be modulated on the order of milliseconds, practically visible as the temperature of the core, which varies on seconds to minutes. They can be used as peaker plants if so designed. In the US at least, the issue is that energy output must be telegraphed 4+ hours in advance.
If I had to guess this is a constructed incentive to make nuclear less desirable than coal or gas under the guise of safety.
The technical ramp up/down issue is often conflated with amortized capital cost.
Nuclear, in order to provide electricity at a competitive rate, needs >90% capacity factor. That means it can't ramp up/down and stay in business, not that it can't ramp up/down. Since the fuel is trivially cheap, ramping down does not provide any economic savings. The opposite is true for an CCGT plant.
... and that is the fundamental reason why there isn't any pressure to build new nuclear in the US (I am less familiar with PPAs and grid regulation in the EU).
If I had to guess this is a constructed incentive to make nuclear less desirable than coal or gas under the guise of safety.