I still subscribe to my crockpot theory that Echo Fighters, despite the branding and push, were still late additions into the game just like clones have always been. Ken being the only exception.
Why would Ken be worked on before the others? Because, perhaps, he will have a small number of differences from Ryu which would necessitate more work than normal. However, because his kit would still be so heavily Ryu, you could still label him an echo and be just fine. Maybe after this Sakurai and the dev team just decide to roll with the punches and add a bunch of clones like Chrom, Dark Samus and Daisy.
Or maybe Sakurai had planned on adding Ken all this time and decided on echo fighters as a concept only after him?
None of the echoes outside of Ken and Daisy have really leaked which I find it curious, and in Daisy's case it can be inferred that she was leaked because people had outside access to the E3 Direct, as I believe she and the contents of that Direct were in a 4chan post a day or two beforehand? Clones have, historically speaking, been very late additions which is why none of the ones in 4 were in any leaks beyond ESRB.