Best case scenario is that this costume was in a finished state almost 9 months prior to its release if they were to release both with the ARMS character which would also mean that we weren't getting fighters like Geno or Doom Slayer at all (which is overwhelmingly why people even cared and tried to position it as real in the first place). The next fighter was almost certainly targeting around a September release date prior to the COVID-19 situation, so these costumes would have been completed for around 11-12 months prior to this leak and at that point 4 full fighters away from the initial leak. Again, in a playtesting capacity 1 year prior to release of a Mii Costume in an already fairly suspicious video that was only testing those two costumes seemingly.
I really don't think it's the best idea to suspend your disbelief to that degree. You just wouldn't be playtesting content like that so incredibly far in advance, play testing is more or less the last thing you do in any development process and you want to do it with the version of the game you're releasing that content with as opposed to a version that will be outdated by 3 or 4 versions at actual release.
The best case scenario is also a fairly reasonable scenario.
The whole "you won't be playtesting that much in advance" isn't really how it works. It was almost entirely meant to come with ARMS in the first place, which has been taking very very long. It's literally only one more character at this point after it was leaked in a close to finished state. The time frame doesn't matter since it hard depends upon a character coming out. Yes, it could mean Geno isn't coming too. Geno could even be a costume with ARMS as well for all we know.
But we already know it can be in testing or held back anyway for well over 6 months. 8 months isn't even that long in this case. We don't know exactly when it could've been licensed. We have no reason to believe them not asking for Doomslayer has any effect whatsoever here, since that's all they said. They didn't say "no Doom content" despite what some have literally tried to paint it as. There's really nothing going actually against this leak that's highly strong right now as a point. That doesn't mean people should blindly believe in it. It's literally a meme that it's real in the Geno thread. It's actually pretty doubted due to the massive timeframe(which as I said, isn't a good indicator of anything. We've seen massive timeframes clearly exist).
Also, Byleth was recorded in November as is. It's quite possible they weren't in a finished state for the entire Mii costume pack planned with Byleth, so weren't thrown in at that point.
So about 8 months assuming the Cuphead dev's weren't contacted yet between then.
While the cacomallow rumour came out in November in an already presentable looking state until maybe June, assuming they won't get a character along with them.
If not then its a bit odd to have these costumes ready this far in advance before a number of other characters, but that's probably just me...
We also have to account for literally it only skipping one character at this point before it'd be ready. So it's not actually that long in reality. Byleth it was unlikely to be completely ready. That leaves ARMS. A timeframe wouldn't matter as is in that regard. It wouldn't have been ready entirely for Terry when we're quite aware how long they tend to be ready before they come out. So it skipping one character? That's not that weird at all.
The months added don't mean anything. They don't release Mii costumes outside of characters, after all.
Or basically, there's no point in looking at timeframes as it's more related to "why'd they skip Byleth?". We don't know. It actually might've not been ready before any of the Byleth costumes either. Byleth was recorded in November, after all. I mean, the fact stuff can happen start 7 months in advance shows already that "it's taking too long" is no longer a strong point at all.
(If not obvious, the new information noted changes my view a bit. I don't think it's likely real, but at least the "taking a while to come out" has a better timeframe that I know of, which means the idea it could be a while isn't unprecedented).