I think Kaguya's byakugan is classified as a Kekkei Mora which higher than Hamura's kekkei genkai byakugan. So I doubt its a question of purity to awaken the Tenseigan and more of a set of conditions to be met to acquire it.Hamura's Eyes have to purer than Kaguya since he had the Tenseigan and Kaguya did not. And she was the original one.
It's a just theory, calm down.
My guess would be just like Hagoromo, Hamura's powers too got split between the Otsutsuki on the moon and Hyugas on earth and the Otsutsuki on the moon needs the eyes of a hyuga to acquire the Tenseigan and maybe vice versa.