We're not talking about criminalizing the behavior, so freedom of speech doesn't enter into the matter. The author is trying to change the market response to the behavior, by pointing out that it has a destructive aspect that many people (particularly men, who dominate the tech industry) may not be conscious of. One reason that we have freedom of speech is that it permits challenges to orthodoxy, and in this case the orthodox position being challenged is that sexist jokes are OK at professional conferences.