I don't believe that Joker was a cram project. I'd wager that his trailer, with all its unique frames, would've taken three weeks at the least to animate, maybe two if you really push past ethical work schedules - but you'd need a long comfortable time to have it ready by the time its non-negotiable deadline came, while contacting both the Japanese
and the English VAs to do a short script. Sakurai's
column on the subject also suggests that the request was neither inconvenient nor unexpected, and we already knew none of the Fighter Pass DLCs had had any work on them done yet, so it's not surprising that any of them would've had gameplay footage ready.
This feels like suggesting King K. Rool was thrown into the August Direct just to throw the leakers off, when there was just way too much ready in his reveal for that to have been rushed in. To Vergeben's credit, he never tried to justify how that one slipped out from under him, so I don't know why he's pushing this idea now that Joker is the first DLC because of his tie-ins to the 2017 Game Awards (which isn't something I'd say really defines Persona 5.)
He sounds more commited to this Square Enix idea, even putting his bets down on the franchise being Dragon Quest. Guess we'll see how that turns out.