Maybe he didn't joke about the endings of the others because the joke didn't occur to him until just then? Why does it have to be some consistent thing he does with the other novels in order to be taken as a joke? I mean would you make the same joke over and over again for the same thing?
The context is "people are asking me what's going to be in the novel so I'm going to make a joke spoiler so I don't reveal the actual twists". It's not that hard to parse. Even the Japanese fans replied to that tweet with basically "lol mothy you're such a kidder".