I'd also argue that it's Venom, not Brock who would be the antihero depending on your definition of that term.
Except Venom and Brock are not separate entities, not in the comics. The symbiote is not Venom; it's just the symbiote. Venom is the gestalt personality that forms when Brock and the symbiote merge. That's why they say "We are Venom." Of course, the later comics have other people (e.g. Mac Gargan and Flash Thompson) take on the symbiote and adopt the Venom name, but that's more just a legacy name getting passed on, like the Flash or Captain America.
Anyway, I think most people would perceive the human lead as the film's hero (anti- or otherwise), not the gooey monster attached to him. And the trailer just doesn't suggest there's anything particularly antiheroic about Brock. It asserts the label without supporting it, so the prominent use of the word in the trailer seems kind of empty. I mean, if he were really an antihero, then you could show that through his words and actions rather than needing to put it onscreen in big letters.