This doesn't sound like the words of someone who is well-versed in the ins and outs of different web server software and their points of difference. It sounds like someone who's just reads the TL;DRs
There are more possibilities than the one you mention. Just because you like one solution, doesn't mean it's viable for every other project.
True. Personally, I like lighttpd, but Dani is an informed person who's intelligent enough to make these decisions. Speed is not the only concern in running a website think security, quick fixes of server problems and so on. Besides which, it would be useless if there was a lightning-fast web server which was difficult, uncomfortable or awkward to use. I'm not saying nginx has these qualities - nginx is fine in my opinion, but a large part of how effective a piece of software will be in an environment is how well the users will be able to, well, use it. Imagine this:
"Grandma A learned to knit with bendy rubber knitting needles. Grandma B learned with traditional needles. Grandma A has observed the amazing ability to manupulate patterns and create garnments which would usually take longer on traditional needles. Grandmas A and B produce equally good garnments, just that one does it a bit faster than the other."
While Grandma A's bendy needles may appear superior, you may forget that they may only be superior while in Grandma A's hands. Grandma B may have great trouble using them and the quality and speed of her garnment production may decrease.
The same applies to something like web server software. Personally, I used Apache for a few years, I got by, I liked it because it was all I knew, but I wasn't able to use it very well at all, perhaps it just never grew on me too much, perhaps I was too stupid to understand its configuration. I used lighttpd and it's the more powerful tool - for me to use, in my hands, it's easier to use, and I can therefore use it more effectively in the situations I do.
Nginx may be powerful in your hands, it may be so in the experts' hands who can tune every last aspect of this or that, but some people may find another tool more powerful for their situation. It comes down to that :)
It should be noted that she is running a variety of web servers for different types of content. The main site is indeed apache for the php instances, but static content is served via lighttpd. Nginx has a lot of potential, the modules aren't quite ready though.
Ah! I see what you mean. I love NGINX, the only thing that is bad I guess to me, would be the lac of .htaccess. :/