I think it's indicative of how well Manchester is doing at the moment that an extra flight to Hong Kong and a brand new aircraft is seen as 'disappointing'.
I have heard the last 2 flights to make it a daily schedule could be added around September time. The A350 is still a relatively small fleet and it was obviously decided LGW would get the extra flights at this time.
It could also be a case that LGW is, itself, getting pretty full in the morning in summers, with very few good slots left. It could well be that CX had a 'use it or loose it' offer and took it, as the early morning slots may be hard to come by in the future?