Is there already a Tendai group called Hoshin Tendai Mikkyo? Just because then it can indeed be confusing. Otherwise, it is not a trademark or something like that. And if one feels like it, why not create a martial arts group with some extras called Tendai Mikkyo? In China there was not just one Tiantai group but at least two that I know of, and they didn't like each other for sure. People can call so many things Zen, Tantra and what not. Why Tendai is an exception? And Mikkyo, well, that is just "secret teaching", not even something restricted to a sect.
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)