The ships just look too contemporary to the Sovereign class to have been built 10 or twenty years previously (which is where their 5XXXX and 6XXXX registries would put them, chronologically speaking).
To be frank, I see nothing
Sovereign-like in those ships. The
Sovereign is characterized by a longitudally stretched primary hull with terracing, and by long nacelles with triangular ramscoops. There are a couple of obvious stablemates to her in that respect, namely
Nova and
Prometheus.
But the
Akira has neither the elongation, the terracing nor the nacelle detail - instead, she looks almost exactly like the
Galaxy,
Nebula,
New Orleans and
Challenger with her white transverse ovoid of smooth surfaces and with her bulbous ramscoops; I see no problems in associating her with the range of registries that characterizes that school of starship design, from the 57XXX
New Orleans to the 71XXX
Galaxy.
All the other ships have the stocky nacelles with bulging ramscoops, too. The
Steamrunner and the
Sabre are obvious stablemates, but stand well apart both from the E-D and the E-E lines. The
Norway might lead to the
Intrepid eventually, but not to the
Sovereign.
Why couldn't all these ships preexist? Virtually every other starship design ever seen has been assumed to predate that first appearance, too - even some "prototype" ones such as the
Defiant have been introduced as old news. That we haven't seen these particular ships before may call for an explanation, but every ship needs such an explanation, not merely the ST:FC ones. And the easy explanation is that we just plan don't get to see many starships. Not until ST:FC and DS9, that is.
If we really want, we can additionally argue that some of the ST:FC types were warships, and we have never seen war in Star Trek before. The more exotic ships might have been stockpiled at Earth for the Dominion war, and were available to fight the Borg for that reason.
Really, the exceptions to the idea that registries are chronological are so few and far between that I have really hard time believing that the ones in ST:FC were "erroneous" in any way, and that the artists intended each and every of those ships to be brand new.
Timo Saloniemi