Good clarification. Depending on the variant of English, "long e", "long u" and "long a" may or may not sound like diphthongs. They can be analysed as /ij/ and /uw/. Wikipedia puts them under "potential diphthongs".
https://en.wikipedia.org/wiki/Diphthong#English https://en.wikipedia.org/wiki/English_phonology#Vowels
Three out of five are diphthongs. "Long E" is /i/. "Long u" is /u/.