I thought if CX were to launch red-eye to Tokyo, it will be HND airport. Hongkong airliners have 2 daily night-time slots (10pm-7am) at HND (I'm not sure if these slots were allocated or not). I assume they thought from-HND-to-HKG red-eye or 7am-departure wouldn't work.

Anyway, in the end, NRT losses a couple of HKG flights later this year: one by JAL, one by CX.