So, as the resident overthinker, this is how I think it's going to go down... E3 is going to be a triple this year, and unlike my triple January reveal theory, this has a ton of precedent backing it up. First and foremost, we've seen this scenario before... A character teased but not properly shown off until E3. We saw this in 2015 and 2018. Lucas was revealed in April and Inkling was revealed in March, we then proceeded to get Roy and Ryu and Daisy and Ridley respectively. Also, if you look at the vast history of E3s with Smash as a focal point, 7 of the 8 had brand new reveals, 7 had at least 2 new reveals and 6 of the 8 had at least triple reveals. The odds of E3 this year being "oh bay-bee, a trep-pull" are really high because of the weird blessing of ARMS being half-assed announced.
Going off this, I'm pretty certain that 6 will be an upgrade and why Nintendo's going with the theatrics of this. 6 will be the very first promotion... And then they're going to immediately follow it up with another. A long-standing theory of mine is that Nintendo was going to promote the most wanted spirit, costume, and assist trophy and I think that's going to happen... But they're going to build up to it. ARMS promotion then the most wanted spirit, then the most wanted costume, and then the most wanted assist trophy at the end or near the end of Season 2. E3 is deliberately going to be the stage for not only the first promotion but the first most wanted promotion based on category. If I'm right, it's G ****in' G, and Nintendo is probably going to save the most wanted costume promotion and the most wanted assist trophy for a separate presentation.
TLDR; Nintendo did this on purpose to make a huge spectacle of the "no promotion" rule getting BTFO to kick off Season 2 and the most wanted spirit will likely be promoted at E3 too.