Once the beef has been turned into a hamburger, it's hard to identify the specific animals that went into the hamburger's composition ... if you know what I mean.
That somewhat holds for non web apps but when you have the source it's pretty easy to tell what it was made with. Usually if it's a compile to javascript language like TypeScript or Dart the source is going to look horribly obfuscated. The js on the page is pretty straight forward so I really doubt he used one of those.
Looking at the source it looks like everything is hand rolled except for the jQuery dependency.
