We're in a bit of a gray area here. Lot of the statements we got early on leave room for interpretation, which is...why there've been so many interpretations.
Granted, Nintendo is choosing the DLC, not Sakurai, so how much input he has for the hypothetical Pokémon we'd get is up for debate. Still, if the DLC for the Fighter Pass hadn't begun until Novemeber 1st at the earliest, I can't imagine why Nintendo or Sakurai wouldn't be thinking long-term with Pokémon. If the interest for a Gen 8 Pokémon was there, they should've made a placeholder and held it off as the fourth or fifth fighter so Game Freak would have more things finalized by the time development for said Pokémon began.
For example,
we don't know to what extent Nintendo was involved. The idea that the Big N must be forcing decisions onto Sakurai, or gave him a narrow list and demanded he choose only from it, was made up, far as I'm aware. I think our instincts are so wired to viewing everything as a struggle for dominance that our default response is to assume one must be happening, but we're best off
not assuming such when we don't have enough evidence to support it.
With the history between Sakurai and Nintendo's higher ups and how good of terms they're on, a back-and-forth collaboration would've made a lot of sense. But we also don't have significant proof of
that, either...which is itself the point.
Since we're mostly in the dark, we're better off considering multiple possible outcomes rather than locking in the first one that comes to mind and deciding it must be true. Jumping headfirst into the first thing that feels real is a logic trap of sorts, and an easy one to fall into...and unfortunately, a lot of the Smash community is a case study.
Fates was actually released in June.
There are some differences between the development behind Corrin than there would be a hypothetical Gen 8 character. Firstly, Corrin was decided merely one month before FE Fates release, so Fates was already out throughout most of his development before his reveal. Secondly, unlike with Ultimate, there wasn’t a set pattern when it came to DLC releases, and there was no set deadline when the team selected Corrin, and Ultimate wasn’t even a thought yet, so they were free to pick whatever new concepts they could get when it came to the last batch of newcomers. This can be seen with the near six month gap between Ryu and Cloud. I feel like it’s more of a coincidence that Corrin was released around the time Ultimate’s Fighter pass ends. This is different with Ultimate as there is a clear DLC release pattern, deadline, and SwSh releases much later than Fates, and I feel that with them using a hypothetical placeholder, they would be cutting it pretty close to that February deadline.
And despite SwSh’s release being closer, many games can still (and have) receive many changes within that time frame. FE: Three Houses getting delayed, and Greninja’s name not being decided despite Sakurai waiting closer to X and Y’s release are couple of examples.
Although it’s not impossible that Sakurai and Nintendo could decide on a Gen 8 newcomer, it seems to go against what they have been doing for years from a developer’s stand point, so I find it unlikely.
And that's the thing, the timing surrounding Corrin is something that isn't analyzed nor accounted for very often. Based on when Corrin's placeholder was added to Smash 4's data, we've reason to believe development on them started some time between mid-April and mid-June 2015.
Since Fire Emblem Fates released in Japan in late June, this means work on Corrin started at a point where Corrin's debut game was either done or
almost done. Corrin actually lines up with everything we've seen since Brawl with
Smash only seriously considering characters if work on their games at the time of the decision is close enough to complete that nothing about them--visuals, power set, or otherwise--could change. It seems Sakurai doesn't want a repeat of how Melee portrayed Roy wrong because they jumped the gun on him. Pokémon newcomers have also lined up with this, since Sakurai has said they waited until X & Y were almost out before weighing their options for Gen 6 and eventually locking in Greninja.
While it doesn't make a Gen 8 pick
impossible for this wave, the fact that there is simply not enough time to wait until Sword & Shield are almost done raises significant questions that a lot of the community has either not thought to ask or dismissed offhand. We're best off trying to get a clearer picture.