Both textbooks I have say that /ʈ/ is indeed a thing (I keep confusing things with the romanizations used
). Even wiktionary lists it in the phonology section on Standard Tibetan. Granted it lists it as /ʈ ~ ʈʂ/.
EDIT: Manual of Standard Tibetan
has a great section on the phonology of Standard Tibetan in Appendix 2, fyi.