you can't regulate the output from nuclear reactors fast enough
From the article:
"The company touts the microreactor’s solid core and advanced heat pipes, which enable passive cooling and also allow for autonomous operation and load following."
Load following to change your thermal output over minutes or hours isn't frequency correcting which isn't grid stability. Steam turbines can't change speed quickly. Some exotic closed CO2 turbine might.
From the article:
"The company touts the microreactor’s solid core and advanced heat pipes, which enable passive cooling and also allow for autonomous operation and load following."