I always figured that the large number of Loki, Ulysses, Zeus, and Herc I fighters you see in the NTF implied that large scale deployment of newer fighters hadn't occurred at the moment of rebellion.
The Loki and the Zeus indeed are probably in the "designed in three weeks" category to some degree, though to what extent is uncertain. They were ultimately produced in large numbers, true, though how large those numbers really were in 2335-36 cannot be said. I suppose you could make an estimate based off of ST and ST:R, though; one might also assume that GTI made off with most of the initial production run of the craft in question. The initial numbers therefore might have been relatively low. However, on THOSE grounds, you might infer that the production capacity for most FS factions is truly formidable for small craft, though the size of larger ships is still restrictive with regards to manufacturing. So... there may be quite a bit of merit in your statement.
...However, what does that signify for the GTVA if that's true? The NTF Rebellion was supposed to be a horrifying battle against extremist government and genocide, yet it took more than 18 months to settle the conflict. When comparing that timeframe to the Shivan conflicts, that's a really, really long time for a fight. Perhaps the following are indicators for why?:
*Refusal to engage in total warfare against one's own species (or allied species)
*Concerns for a more balanced economy or other such concerns; in the case of Shivans, the economy and social issues suddenly becomes far less concerning...
Either way you slice it, it's very strange behavior for the GTVA, and the points above might be very useful to consider when writing a campaign. Alternately, straying away from your theory one might consider this:
"New" fighters were in production and service, but the majority of which were with frontline GTVA units. Some of those units defected to the NTF, certainly, but the percentage and composition of the defectors cannot be ascertained. The majority of ships available to the NTF would have therefore been ships placed in regional storage, most likely the older types you mentioned. You will recall that both the Herc Mk.II and the Myrmidon were in service with the NTF, though in much lower proportions from the perspective of the player.
Therefore, as per age, I think it's very reasonable to state that both the Herc Mk.II and the Myrmidon are at least a few years old. The Perseus is canonically quite new and was just getting ready to enter service during FS2's main storyline; the Valks must have been phased out of most or all front-line squadrons during this time, and the only reason we saw none of the latter or the Athena bomber with the NTF is that those files were not included with the game!
Continuing this, the Pegasus is new (though if its development was started earlier or for other purposes could be argued?) and is stated to have been developed to operate specifically well in nebular environments... so, maybe just two weeks that time? The Erinyes seems likely to have been another "three-week fighter (though it was there during the first SOC loop)," as is the Ares and the Artemis D.H. Of those, the latter two only came into service by means of a new engine from existing (aero)spaceframes, and so may not be as new as they seem (in a way). As per the bombers, both the Artemis and Boanerges are said to have been of the same technological generation, so the NTF Rebellion may have been a factor in their development. Otherwise, all the other bombers were at least 30 years old and a replacement was due.