Sexism definition: Prejudice, stereotyping, or discrimination, typically against women, on the basis of sex.
This was clearly sexism since it was demeaning to the woman and her gender was used as a part of that (e.g. the fact that pictures of naked women were used in the attack).
If the same happened to a man, we would probably call it harassment, not sexism, since men at large are not discriminated against based on their gender. A female posting pictures of a man at the conference wouldn't be signaling that men don't belong in the group, since the group is dominated by men to begin with.
Either gender can be stalked, but there are FAR more female victims, a symptom of sexism at large in society. FAR more women are harassed as well. 1 in 5 college women are raped during their college years.
To take some of your words, the deed was encouraged by and reflective of rampant sexism in society at large.
I don't think I've done a good job explaining this, but hopefully provoked some thoughts on it at least.