I suggest looking at the header. Look for meta tags and for scripts from other sites. Try to make a quick look at first. I made a quick look and I found the following in the header.
That might not help but that is the type of thing to look for. It might be what is being used for the pop-ups.
The builtwith.com site might help but … wow … there is so much stuff there, right? It is all called called usage statistics. There might be nothing there telling us what is used to build the site.
If it is only a WordPress plugin then that seems to be the answer. Or perhaps I misunderstand the question.
Why do you say it is a WordPress plugin? The OneSignal website says nothing about WordPress. I believe you that there is a OneSignal WordPress plugin but I doubt that OneSignal is only a WordPress plugin.
I’m not sure what they are using, but once thing is certain is that they aren’t using Wordpress. It’s also running off IIS, which could be a lot of things but it’s probably ASP.NET.
Okay, sorry. Now I see. The usage statistics is a link to the use of that tool in all (tracked or whatever) websites, it is not saying that the tool itself provides usage statistics.