But they don't use any of that, do they? They don't need to at all.
Though it helps that the Koopa Clown Car exists and is a core of the moveset, which is how he was able to easily mash them together. Though to be honest, the ARMS aren't much different in the analogy, as they represent the main unique focus of the characters, with them being combined otherwise with small combat abilities. Not unlike Simon and Richter(just as one full character instead of two).
The thing is? The ARMS characters are actually pretty similar to this situation, just don't need a vehicle. There's enough with proportions similar enough to easily put them together, with voicework to keep their personalities intact, at most their taunts could have slight differences in animation but no tangible gameplay changes(like Byleth), while the core thing is they can use a ton of unique ARMS in general to spice them up. Also, we know that having slightly altered models can work for an alternate costume as shown with Ridley. Stuff like armor isn't the same as entirely different proportions. I also wonder if part of the delay is slightly reworking some models to better fit. COVID19 is obviously part of the delay too(...well, and told to us, but it wasn't exactly a hard guess either).
I'm not saying this is favorable to people, just actually pretty realistic as a scenario.
(Incidentally, I wish they would just give Alph some alternated taunts akin to Byleth to fix his personality. This is seriously the only issue with him being an alt. He's completely OOC in comparison. At least the gameplay is accurate otherwise).