The creator is taking advantage of the fact that most of us read words semi-holistically (if not, it's a hell of a coincidence). Research has demonstrated that as long as the first and last letter remain unchanged, we can read passably well. (Though "bookmarklet" took me some time to decipher.) Difficulty does varies from anagram to anagram, however.

I guess the idea is to mimic the increased cognitive load of dyslexia to give non-dyslexic people a feel for it, without making each word as difficult as deciphering full anagrams.