They chose to "refuse to run the more extreme ads" that were well within the bounds of moderate Labour policy of relatively recent years. With a manifesto that resonated pretty well with voters on most issues as it turned out yet didn't include several of Corbyn's actual well known fringe views such as unliatera disarmament as that went against agreed party policy. The manifesto is what counts.

Thus he's being treated as some dangerous extremist whose views can't even be let out in public during an election campaign. Views that, as put forward in the manifesto, are not actually that extreme. It's miles away from Foot's longest suicide note manifesto for example.