Why did Linus make such fanfare about his departure if it was always intended to be so short?
I'm not sure he did. AFAICT he wrote an email on the Linux mailing explaining that he was taking some time off and why. He also did an interview or two, presumably because he was asked.
He probably realised that a portion of the internet would go bananas but that's not entirely his doing.
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.
But, I suppose for someone who's always quite involved with everything and doesn't take long breaks- one month can seem quite long.
That said, I also wonder if he moves too quickly, or if he found an agreement with other FOSS leaders.
Check out this discussion for instance: https://news.ycombinator.com/item?id=18281676
Also note that my previous comment is at -4 already of the time of this writing, which must be done by someone who doesn't like that opinion. You wouldn't go ask 5-7 of your friends to give downvotes to that comment just because you disagree with it, right?