Don't know how they came to this conclusion, if you look at the headers of https://twitter.com its listed as "tfe".

  $ curl -v https://twitter.com
  [..ssl handshake and request..]
  < HTTP/1.1 200 OK
  < content-type: text/html;charset=utf-8
  < date: Fri, 01 Nov 2013 11:12:31 GMT
  * Server tfe is not blacklisted
  < server: tfe
  [..body..]

Also interesting to note that they are identifying sites such as "github.com", "dell.com" and "starbucks.com" as all using GWS[1].

[1] http://trends.builtwith.com/websitelist/Google-Web-Server

But if you actually click on the dell.com profile, it doesn't. It seems they have some data issues.

We've excluded most websites which contain user generated content because of the false positives they give off like this. We've added twitter.com to that list.