Keep this in mind https://support.google.com/webmasters/answer/183668?hl=en&to...

> Break up large sitemaps into a smaller sitemaps to prevent your server from being overloaded if Google requests your sitemap frequently. A sitemap file can't contain more than 50,000 URLs and must be no larger than 50 MB uncompressed.

The site is built on Tumblr which automatically generates a sitemap for the individual posts, but not any other pages on the site. For example the "about" page is not in the sitemap, but is still indexed.