How adding a capacitor reduces the signal gain to 1 in an op-amp?
Here I can't understand how adding a capacitor reduces the signal gain
to 1?

The text you quoted is talking about \$\color{red}{\boxed{\text{DC only}}}\$ and therefore, putting a capacitor in series with R1 basically open circuits R1 (at DC) and, you are left with a simple unity gain non-inverting amplifier: -