Sarwate et al.'s lower-bound conjecture for binary m-sequence cross correlation

Let $n=2t$ and $p=2$. Define the cross-correlation quantity $\text{S}_d(z)=\text{C}_d(z)+1$. Sarwate et al.'s conjecture. One has

$$\text{S}_d(z)\ge 2^{t+1}.$$

The paper states that this conjecture is confirmed for the binary decimation considered there; its general status is not specified in the supplied text.