I don't think anyone doubted he'd be a big contributor, as he was a big contributor in Miami every year. But to act as if people were dumb for thinking he wasn't a deep threat is ridiculous. He's never shown an ability to be a consistent deep threat in his career, and based on his combine numbers he might be the worst athlete in the NFL at his position (looked it up and he literally has 0th percentile agility and burst). Those numbers combined with his career production thus far don't really point to him being a deep threat, but hey, Antonio Brown's also a pretty shit athlete by NFL standards and he's the best WR in the league, so sometimes you just don't know.
Not gonna complain if he turns into Antonio Brown, though.