Your understanding is not quite correct. The drivers are wired in opposite phase, but they also face opposite each other so they are actually mechanically in phase. This is what the 'push pull' design is - one driver pulls when the other one pushes, etc.
From the following page:
The Vance Dickason that the author mentions is actually a famous writer of speaker design textbooks. I have a copy of one of these textbooks for my DIY speaker projects. Tomorrow I'll pull up what he has to say about push pull driver configurations, but I think it should be clear that having the driver wired out of phase is a deliberate attempt to overcome distortion products.
Another prominent example of a push pull design would be the Sonos SUB, which I think you might have all seen in electronics stores. It's interesting stuff.