^My understanding is that a new ship model was designed, but the producers didn't like it and requested the D7/K'tinga be used instead. Since producers are above art department in the grand scheme of things, I'd say it was a deliberate choice and not an error.
Besides using the NX class as the top ship still dosen't make sense even if there is no Constitution class since they still have what ever class the Kelvin was.
I took the premise to mean that something caused the Feds tech to stagnate at the ENT level, so no Kelvin ships either (Einstein-class, according to Federation: The First 150 Years
We know from "E2" that those NX-class ships are built to last.