In fairness, Byleth is the only actual unique FE newcomer this time, with the only other one being an echo. Everyone else has been brought back from the last game (including certain that are fan-favorites, ironically enough
), so it's all cumulative. Granted, it's what the network wants, and whatnot...
Also, it lends credence to the silly theory that Nintendo's easing Sakurai into doing stuff they want by starting with characters that are almost entirely from stuff he likes.
It also explains the occasional gaps of info we had, Sak was busy playing through an preliminary pre-release version of TH, so unlike other games he's played most of the stuff in there was unfamiliar to him (which had to be quite the challenge in this case). When the character he works on is from something he's played that's less recent, they're naturally much quicker to implement and start development for.
Finally, the end of the article notes that they're in the process of developing the next 6 DLC fighters, something that'll be brought up in the next column in two weeks.