In control systems the loop gain LG (product of all transfer functions within the closed loop) is a very important parameter.
In your case, the loop gain is LG(s)=G1(s)*G2(s)*H(s).
As you can see, the closed-loop transfer function for the disturbed input
will be rather small ("almost zero" in the text) for a large loop gain LG(s)>>1.
For the reference input the situation is different because the product G1(s)*G2(s) appears also in the numerator of the closed-loop function Hr(s)=Cr(s)/R(s).
With other words: Both closed-loop functions have the same denominator (1+LG) - however, the numerator for the closed-loop reference function (G1*G2) is larger than for the closed-loop disturbance function (G2). Hence, the influence of the disturbance signal is smaller if compared with the reference signal.