I think you've made an important distinction here. An action is sexist if it is a material discrimination against a person because of their gender. However, there are emergent patterns in environments with large gender-asymmetries which cause one gender to have a very different experience than the other.

One man making an advance on a woman might be okay, but several making the same advance on the same day is going to become very uncomfortable. Do we call this sexist? No individual is making any action to cause discrimination, there are simply environmental factors that create an imbalance.

I think a lot of the issues surrounding women in tech are emergent from this asymmetry, and not strictly "sexism" as I have defined it above.