"When followed or preceded by a vowel **but not word-initially**, /i/ shortens and forms a diphthong with the adjacent vowel. Examples: sigie [ˈsid͡ʒi̯e], perstelitio [persteˈlit͡si̯o]"
is at odds with
"i in intervocalic position or word-initially represents the semivowel /j/. Examples: iaro /ˈjaro/, bluiare /bluˈjare/."
Phonetically, there is no difference between [j] and [i̯]. Whether it is analyzed as an instance of a phoneme /i/ or a distinct phoneme /j/ depends entirely on the phonological analysis.
This description seems to imply that there **is** a phonetic difference, upon which the phonological classification (i.e. /i/ vs /j/) seems to be made.