That isn't irony.

There seems to be a tendency in the nodejs community to try and use it in any and every situation, regardless of how suitable it is for that situation.

Yeah not really ironic; I mean I would be surprised if node.js wasn't a decent or maybe even a better fit but perhaps they had more technical expertise to handle PHP development and the best way to try and start a business is to release early and often.

In general, how do you determine what software is running on the server side? None of the obvious links are to .php documents and the headers only indicate nginx

Look their cookies. You will see PHPSESSID meaning that session is provided by PHP.

Although, you can do the same with node session module it seems like php anyway.

Plus: http://builtwith.com/evennode.com

There's a cookie called PHPSESSID.

A PHPSESSID appears in the cookie.