The most likely explanation for not using Swift is the employees who worked on the crawler simply didn't know it existed. From what I've heard, only a select few at the company knew it was in development and a number of Apple employees were noticeably shocked during the WWDC keynote when it was announced.

Apple actually uses a bunch of 3rd party developer tech on the server-side and on the web. They use Rails quite a bit I believe and are starting to use Angular.js too.