This should be your trigger: "...LON-HKG shows I4-I9 for availability, even for 2 days out...". I Booking class is seldom (or usually never) available shortly before the flight date, but now it is.

This implies the airline is prepared to sell tickets in any booking class, and not only the most expensive J booking class one, as is usual, shortly before the flight date. This in turn implies they don't expect to sell substantially more tickets before the flight takes off, so are just dumping availability to fill up the seats and keep up the impression, the flights are full. Having the additional promo may help potential buyers to choose CX.

Or so to say, until recently, there was a suitable high-pricing ticket sales level, which now vanished. Read the signs.