Except you can't. Because it was barely a tiny bit before he appeared. The real fact is Joker was already selected in Smash way before his game appearance would exist. Remember, the game came out about a month before Joker was directly announced, even before Smash Ultimate would release. There's also no telling exactly when it would release, making it a non-factor at that point. The official reveal of PQ2 was in 2017(though that was more of a light preview, as technically the official reveal is treated as a year from then. Do you honestly think Joker was selected in August 2018, despite being noted in development for a few months before November 2018? As I'll note before, the timeline just doesn't match up in any way) as well. Beyond that, Joker was actually delayed in development due to Arsene being harder to work with. They didn't even have enough to show immediately on his own reveal in December, a bit before Smash Ultimate released. To quote; "As the first DLC fighter he should have been the first DLC fighter to be developed. He was actually developed a few months before the game went gold and we had to design him during our most difficult development period." Now remember that the game went gold in November. He started development as early as August or July(the term few is used here). This means he'd have to have been licensed before August at the earliest, before the actual proper announcement of when PQ2's date is. It doesn't necessarily mean that Sakurai was unaware of the release date, but it also means it couldn't have been a factor in why he was chosen anyway. See below about what the whole point of Nintendo appearances are. And hint; it's not about "it just happens to be a character releases in the game after a Nintendo appearance", as that's a major misreading of the actual argument. It's why it's not the actual argument used nor has any real meaning if it's not actually held up to proper details. Anybody can luckily be in a game right before they're made DLC. It's a coincidence. But actual merits of these kind of appearances are the factor that it happens before they were chosen. That's what makes them "eligible" in fan circles. Now, what really would be applicable instead in reality of development is that Joker was already intended to be in a Nintendo game by Atlus, which means adding him to Smash is even easier by then, since it lines up with both Nintendo and Atlus' interests.
Something else to note; DLC for Smash Ultimate was already greenlit(or more so, it was Nintendo's idea) in July 2017. Now, this is where it gets interesting. While Atlus actually noted Q2 was a thing that existed in August 2017, the release date wasn't shown until August 2018, and it wasn't till November 2018 it would release. The fact the date was announced extremely soon from it implies they weren't sure when it was close enough to done. While it's possible the release date was known enough by Sakurai, the other data barely matches up. Thus, the eligibility factor has pretty zero meaningful impact here in practice.
Joker, in the end, was eligible by being a video game character from a very big game. Being that Persona 5 wouldn't even be brought to the Switch at all, it's really a stretch to say a spin-off game is why he actually got in. Despite quite literally zero spin-offs are actually acknowledged in Smash, while instead, P3, 4, and 5 are. Which mind you, were not on Nintendo. Maybe it goes to show that it wasn't the Nintendo appearance in particular that mattered. But moreso, Atlus cooperating with Nintendo that made him an easy pick. They have been cooperating even earlier, with the first PQ in 2014. Which means Persona had its door into Nintendo, or in other words, the franchise itself would play a bigger role at that point than any lucky appearance before Ultimate.
To be even more clear, when people talk about "appearance before Smash", it's not about simply appearing. It's about appearing early enough to already have a Nintendo appearance in order to be eligible. And let's be real, there's no way PQ2 is actually what made him eligible. The dates don't match up remotely for that to make sense. If it was from P3 or 4, absolutely, as PQ itself was released very early, before 2015 itself. It's nothing more than a simple coincidence at best. Joker broke the trend of eligibility, basically. He proved you didn't need a Nintendo background to be eligible. What he didn't show is that your franchise having Nintendo relations is unimportant(or if that's hard to read, even for me when first typing this, is that Nintendo relations comes down to the overall franchise first and foremost, not strictly the character. And that's iffy if it matters all that much. Cause, well, company factors play a role even moreso. Licensing also is vastly important, though things like fan demand and how big a franchise is also matters. And let's be real, what made Atlus say yes is pretty easily a thing called cooperation. What made Nintendo say yes is pretty evidently how well Atlus is cooperating, with tons of games planned out, not some ill-defined eligibility by that point(which has never been stated to outright requirement a Nintendo appearance in any form).