Did Serenity really appear in Battlestar Galactica?

Yes, that's Serenity. Sadly, there does not appear to be an in-universe answer.

Jarrod Davis, who was a digital artist on the episode, revealed on Facebook that he put Serenity in the show on the advice of CG supervisor Lee Stringer:

Transcript:

So we on the Crew were all about cameos, and I do believe it was Lee Stringer who had the idea to put the Serenity in the show somewhere. It fell to me because I had these shots in the hospital where Laura Roslin gets her cancer diagnosis with all these windows that needed filling with spaceshippy goodness. Perfect opportunity of course.

So I do it. And you've seen it. And I think, hey eventually some nerd will see this and get a chuckle out of it.