This situation is... Well, complicated. But as a huge freaking nerd connoisseur of fantasy and science fiction, I can have some knowledge of this protest - and why there was a counter-protest that contained Antifa.
Anyway, to understand it, you have to go back to the long long time ago to the year 2015. A conservative sci-fi writer was felt that the nominees of the Hugo Awards were based on the fact of minority status and not their merit as an author. It would be a fair enough complaint but they decided to manipulate the nominations and it caused a controversy.
Anyway, this group of conservative fantasy fans and authors was called the "Sad Puppies" and one author technically broke off the form the "Rabid Puppies", although the two groups of worked together a lot. Anyway, the dude that lead the Rapid Puppies, Vox Day was a real piece of work. A supporter of the 14 words, called a black woman who called him racist a savage, etc. Basically, he's the sort of person that Antifa would protest and since the Sad Puppies never told him to fuck off, there's a bit of guilt by association.
Anyway, the Sad Puppies were quiet for a few years until they decided to protest what they saw as discrimination against conservative writers of fantasy and science fiction. Since the average fan wouldn't really care all that much and the press less... Well, they needed to sensationalize. They decided to rebrand it at as "Anti-p***phile" protest, calling out Walter Breen (who was actually banned from at least one sci-fi convention when he was accused of child molestation), Marion Zimmer Bradley (who was accused post-humorously, took a huge hit to her rep, and had her publisher donate all the profits of her books to anti-child abuse groups), and Samuel Delany (who I admit to knowing nothing about). Basically, making a whole lot about nothing and put a more respectable from to what basically amounted to people mad that they don't get the respect they feel they deserve.
It really had nothing to do with "anti-p***philia". That was just a rebranding so they could get more attention, nothing more. In reality, antifa was likely there because they were Sad Puppies, which willingly worked with an outright racist person to advance their goal in the past, and likely saw through the anti-p***philia part of their protest as being a smoke screen.
But I digress. Antifa wasn't there protesting them because they were anti-p***philia. They were there because they were part of the new wing of far-right fantasy/sci-fi authors who are mad they aren't getting attention in a genre where they were never really part of. (As a whole; there have been some good conservative fantasy and sci-fi authors but in general, both genres have always had a liberal lean if not outright leftist.)
On a side note... Holy shit, that took some time to actually look into.
