This seems to make sense. I was tracking the actual passenger loads on all of the Australia routes over last summer / fall (US summer and fall) and many of the flights went out with empty seats in J and a half empty Y cabin. I do question the late October restoration of daily Sydney service. Last year, flights were beginning to fill up by the end of September. I would think at least going 5X weekly to/from both locations by the middle of September would better match the gradual seasonal increase in ridership.
I also wonder if they're expecting the SFO-MEL flight to cannibalize existing SYD service. Either way, since IAH-SYD was 5X weekly last summer, the net loss of total AU service is only 2 RTs per week.