The Valk being very recent is fanon, fanon that makes some sense though...
While they didn't outright go and say "This ship was never in 1CW", I wouldn't call it fanon - I'd call it common sense. Here's what we do know about her:
- much smaller than Galactica, with less firepower and smaller air wing;
- Adama used to command it, then got reassigned to Galactica as a form of punishment.
- the Valkyrie was apparently considered a pretty good command.
- you'd typically send a modern ship on a stealth black ops assignment if you can help it, leading to conclusion that the Valk is small, but state of the art. Especially during peace time when your fleet resources aren't stretched thin and you can choose.
- the exterior and CIC resemble more modern battlestar lines, and compare well with the Pegasus style.
Basically, if the Valkyrie was also over 40 years old at the time of the fall, then how is reassigning Adama from a smaller 40+ year old ship to a larger, more powerful 40+ year old ship a punishment? Feels more like a reward to me. Sure, the Galactica was about to be decommissioned, but how much time would a 40+ year old Valkyrie have ahead of her at that point anyway?
I'd say this is yet another example of just throwing everything in without thinking it through. I guess we could pretend the Colonials used the same Valkyrie space frame because it's essentially good, and just kept updating it's systems and weapons over the years without too much external differences. So the old Valkyrie we saw here could be the original, and the ones in nBSG could be like tier 4 refits or something. That would explain almost everything apart from the modern lines that seem to suggest "modern light battlestar", not "1CW slugger".
And no, there's no way you'll defend the horrible, reflective hull Osiris. It makes the Berzerk look good in comparison