Yankel, you mean YD 242:2? And then 244 says that a hoshuve rav is like your teacher. It seems to carve exceptions when not in front of him. Also, this chosid might not have felt that he is bound by R Moshe’s psakim, but rather by his chasidus, so then the logic of YD 244 does not apply.
See also Kesubos 85b for when Tuvia might or not include Rav Tuvia.
Again, I am with you in terms of being respectful to Rabonim one disagrees. I do not like especially when this happens b’davka, ie using multiple titles for rabbis from your side and mr or dr for those one disagrees with. But I don’t think occasionally omitting titles of well known Rabbis or those who have pen names is a bog deal. For example,we say Chofetz Chaim, not Rabbi Yisroel Kagan.