@Ravenstormchaser
If the adventure was already published, I'd agree with you. There are hundreds of ways that this situation could come to pass. However, none of them is written in the adventure - and there's still time to change that. It's not that the battallion can't be explained, it's that it hasn't been explained. The fact that it has raised so many eyebrows here should indicate it will raise many more in the future, even though it is merely an oddity, a throw-away reference that is never picked up again.
It's like if I wrote a murder mystery set in the real world, described one of the characters as "Oh, and he has blue skin" and never got back to that. My readers would at first wonder why he's got blue skin and then, when no explanation follows, settle for "Because the author wanted him to be a special snowflake. Ugh." And that's what I feel is happening here due to the rarity of Landraiders. Either explain them or drop them.