I expect it's more fans connecting dots than it was any kind of forward planning. Discovery and Picard haven't been working together to set up a short YouTube video by OTOY.
Yeah, this is the kind of thing the Trek novels have been doing for ages. The shows drop in little details that seem interesting at that moment, and if you're interested in making a new story that builds on that instead of something more disconnected and episodic (the way mainline Trek historically has been), you'll have plenty of details you can pull up. And given that this is all being done trailer-style, very impressionistic, it's probably not hard to drop in new ideas and sequences to expand in different directions. The story in this short probably came about after PIC season 3 revealed Starfleet's Zombie Avengers program,* but it's incorporating aspects from their earlier storyline(s), specifically the adaptation of Colt's trip to the future in the comics.
Back on the A/V Club, someone called it "plot jazz" when it was done on DS9. They just had a lot of stuff going on, and had a great writing team who could craft long-term storylines that seemed pre-planned even though they were working as they went (and now that we've seen more about the early days of Babylon 5, for instance, we know "pre-planning" isn't always entirely what it's advertised as).
*Speaking of, I think the actual use of the Starfleet Zombie Avengers being to let two people have the personal goodbye that fate denied them is a lot better than any kick-ass, epic, Kirk-versus-everyone crossover event, and for nothing else, that zig instead of a zag makes this short remarkable and, hopefully, precedent-setting.