Theory on the moons.

[Inaaca]

And in this fictional world it's totally possible that the inhabitants always believed the 2nd moon to be a moon, but may later find out that it is actually something else.. Like a planet, (or meteor).
What Cairdeas said makes sense.
In the lore they're only called moons because the people living on the planet believe them to be moons. They could be wrong. Even with the lore this would make sense. I hope I explained this well enough.

As an example.. we've called Pluto a planet for so long. Now we're being told "oops, we were wrong, it's actually something else." It's not unrealistic to believe that the same could happen in a fictional world.

Never said it was impossible or unrealistic, just very unlikely, that they would falsify this kind of information.

From the perspective of a game designer, I don't think they would build the entire basis for an important deity on a chunk of lore that isn't even true to begin with.
I also don't think they would present false information about the basics of the world we're in, when all they've been doing lately in quests is trying to give us more concrete information and/or lore about it.

Also, as far as the Meteor theory, I think this is likely and doesn't conflict with the twin moons, considering a meteor that's been orbiting Hydaelyn can still technically be considered a moon (or satellite, I suppose).