There are more forms of discrimination on the basis of sex than unequal pay, which is what this survey most directly evaluates. Some of the forms of discrimination are illegal, others are not. Some of the latter may be due to cultural effects which we collectively may decide to change.

The problem is that logic you gave is suspect to a survivorship bias. Consider a hypothetical where 50% of the women in tech are arbitrarily excluded from the field at age 30, while none of the men face that exclusion. The men and women who are in the field at age 40 have the same experience, education, and pay. Thus, equal pay alone does not indicate the lack of discrimination on the basis of sex.

As a more real-world example, Robinson v. Jacksonville Shipyards, Inc. https://en.wikipedia.org/wiki/Workplace_harassment#Robinson_.... is one of the earlier court cases which established that sexual harassment can lead to "an increase of women quitting jobs, getting transferred, or being fired" and "The sexualization of the workplace imposes burdens on women that are not borne by men". (Quotes from the decision at http://www.leagle.com/decision/19912246760FSupp1486_12018/RO.... ) If more women welders leave the field than men welders, because of sexual harassment in the workplace, then it doesn't matter that those women who stay in the field have the same pay as the men who stay.

Thus, the "big if" might be expanded to "if men and women are given the same encouragement, same access to education, and same opportunity to gain experience." This survey assumes that that is true.