I do not have access to the article you are citing but iI have made a little search and iI think the answer to your question is yes (given that we are speaking about a non-compact, connected, semisimple Lie group $G$).
You can find the proof in a previous article by the same author. See: Kaehler coherent state orbits for representations of semisimple Lie groupsLisiecki - Kaehler coherent state orbits for representations of semisimple Lie groups (it is proposition 3.3, p. 251).