And then the Colossus takes the field, and it's game over.
Yes. For the Colossus. Given how good UEF ships are at subsystem sniping, the Colly itself can be rather quickly neutralized as a threat. Sure, she'll still have that 1 million hitpoints that the UEF can't really deal with quickly, but her main armament is a lot less threatening.
Also, you are making several assumptions not backed up by anything. One is that beam jamming would be unavailable. Two, that the UEF actually would go on a full on offensive without logistical preparations. Three, that the raw in-game stats as seen in the tables tell the full story.
...beam jamming
wasn't available initially in WiH's events (or rather, the end of AoA), the game itself stated that, I think. 18 years prior to that, I'm pretty sure they still wouldn't.
If the UEF went on the full offensive with full logistical preparations, their entire military would look and operate totally differently. That kind of defeats the whole point of the exercise.
The Colossus isn't anywhere near that vulnerable. Even if you factor out the psychological and morale factors, the remnants of the UEF fleet are being engaged by two destroyers, at least several corvettes, and at least a dozen cruisers, along with dozens of fighters and bombers.
BGreens, especially if overdriven, are horrifyingly powerful against UEF ships (with the sole possible exception of a Solaris). Add in supporting fire from slash beams, bombers, and the damage they took previously, and it's a one-sided fight.
Factor the morale and psychological aspects back in, and things would go downhill for the UEF very quickly in that situation.
The Colossus has 240 craft to defend its subsystems, intercept enemy craft, and even disable/destroy enemy ships. And its point defense systems aren't lacking, either. UEF ship weaponry is slow to damage unless a ship's forward-fixed gauss and mass driver cannons can be brought to bear, though it's questionable whether or not they'd make it that far, or succeed in time.
I've looked at quite a bit of raw stats, and these are the impressions I get; the UEF is not that good at fighting a war, especially an offensive one, while the GTVA shows rather incredible ability to adapt, plan ahead, and be prepared for many scenarios. The UEF also lacks much meaningful combat experience; the GTVA has plenty of it.