I haven't seen any marketing material from Nginx, but I think there are historic reasons for these perceptions.
I've personally experienced the problems with Apache and prefork. We couldn't switch to the event model because modpython/modwsgi wouldn't work with it. And prefork was really awful when it comes to serving files. Nginx also had much better defaults. So you could use the version from the distribution and bombard it with requests without it breaking a sweat.
In the end, I think convenience is really overlooked when it comes to server software, or at least has been. Most shops don't have lots of time to spend on tweaking things. The defaults must be good and necessary configuration should be simple as at all possible, autodetecting as much as possible.
Not that nginx is perfect, the caching layer is one example of something that doesn't really do the right thing out of the box. Last time I looked, there was still a bunch of really common stuff it doesn't understand by default, e.g. it can get confused by compression headers or common tracking cookies.
On a similar note, I once sent a bug report to Varnish that didn't at the time respect Cache-Control: private out of the box. In the end it was wontfixed by PHK.