She led a political movement in direct opposition to the incumbent that established her name and her causes with the people in the city to the point wherein this poll shows her 9 points ahead of Emanuel. For a start, does any other possible candidate have a higher name recognition, or direct identification of the name with a politics? Is there any individual better known as the opposition to Rahm?
But you are right: July is very, very late. It didn't used to be. (Not even in 1992, I remember Perot (re)entering at that point.)
As for having no money yet, that can be jiu-jitsued as a plus. In effect, Rahm's inevitable flood of attack ads will serve as the reminder to vote for her. This is what Bloomberg managed in 2009, when his opponent was a nullity. No one with a chance dared to run against the bastard despite the coup with term limits, because said person would be chopped up and cut out "forever" if they lost by the merciless Bloomberg political machine. We were getting mail every day and Bloomberg commercials several times an hour, in a barrage that had already started a year in advance. He spent like $180 million out of pocket (and why not, his fortune grew from $2 billion to $18 billion during his tenure) and hired unbelievable numbers of street workers to maintain the harrassment. This proved highly successful in getting people to hate Bloomberg every day, and bang! On election night the nullity had somehow come up with 46% of the vote, thanks to Bloomberg's own campaign of demonizing him three times an hour every hour every channel on the telly. (If certain people had figured this out we might have had, gag, Mayor Weiner!)
De Blasio also played well with a lot less money than Quinn. It was about strategic and intelligent application of an ANTI-MONEY message (whether or not it was sincere), which 73% of New York voters were ready to hear.