One thought I had is that they swapped Chun-Li for Ken early on after they realized they wouldn't have enough time, and then decided "Hey, what if we made more?" a short while after that. The Echoes are quite low-effort, with Chrom in particular feeling rather awkward (Still in Robin's Final Smash, not having the Awakening victory theme until an update, etc.). Admittedly, it's kind of a stretch; Mostly just a bit of speculation on my part.
Though, it does raise the question of when in development Echoes were created. Were they going to be a part of Ultimate from the beginning to help the reception of Dark Pit and Lucina, or were they added later to squeeze in a couple extra fan-favorites? Not sure we ever got a statement on that, though I could just be forgetting it.