Earlier quoted context omitted.
To me on the other hand, it was obvious that he would be back as soon as 4.19 was released and the 4.20 merge window opened (the kernel release cycle has two phases, the merge window where new features and the bulk of the changes come in, and a phase where only bug fixes are supposed to come in). He took advantage of the time when the merge window was closed, since it's a less hectic period, and handed over maintenan…
That's impressive foresight. Why did Linus make such fanfare about his departure if it was always intended to be so short?
Not really, it was on the announcement email from Linus (https://lwn.net/Articles/764901/):
"To tie this all back to the actual 4.19-rc4 release (no, really, this _is_ related!) I actually think that 4.19 is looking fairly good, things have gotten to the "calm" period of the release cycle, and I've talked to Greg to ask him if he'd mind finishing up 4.19 for me, so that I can take a break, and try to at least fix my own behavior."
If Linus asked Greg to "finish 4.19 for him", it's predictable that he'd be back for 4.20.