Firstly nginx is a web server. Its a reverse proxy used to serve content to the web to be more specific. It is not a virus that infects the iPad.
Usually when you see the nginx page that means in this case "your favorite site" was temporarily down. You said the site was inaccessible and then it became available again. This seems like a situation where the web developers had some software issues or networking problems to be addressed and once they were fixed everything worked as expected.
This doesn't stick out as a typical case of malicious software rather human error albeit intended or unintentional. As a software developer I use nginx on a daily basis. I was looking for benchmarks on caddy and nginx and came across this Q & A.... Suffice to say I couldn't let this go unanswered.