There's whole threads dedicated to arguing over that. They didn't technically say that. They said the first two are exclusive but they didnt say only two, so it is possible this one or others may be one month exclusives as well.
The official Xbox magazine, which I'm fairly certain would be in the know regarding this sort of thing, also reported 2 DLC. So arguing semantics is kind of stupid.
One thing I've noticed, when Bethesda-
published DLC are exclusive, they usually have the 360 logo at the end of the trailer. Dragonborn didn't have it, but Dawnguard and Hearthfire both did. When New Vegas had its DLC come out, Dead Money, which was exclusive, had the 360 logo. Honest Hearts debuted on Steam the same time as LIVE, and it didn't.
Another thing to consider, is that all of the confirmed exclusive DLC were identified as such in press releases- no such announcement has been made for Dragonborn.
So if past experience is anything to go by, it's
probably not exclusive. It is strange they didn't mention a release date for PC, but they have said they're working on it. So it could be the PC version simply isn't ready yet, or it could be any number of other reasons.
There's no reason to jump straight onto the exclusive bandwagon when there's no real proof.