Except that some of the bonus kickstarter tiles modify troop tiles that have no value rather than a 0.
The correct interpretation, for all tiles, is that if a tile has a zero (or a higher value) it can be modified. If it does not, it cannot be modified.
This is also true for the extra content. However, the small rules sheet supplied with the content bundle says "+1 to attack and defense" for two tiles, and this is a misprint. Instead, both of the descriptions for those tiles should have read "+1 to Attack".
Fortunately, the main game rules and the affected tiles themselves are correct. We apologize for the confusion caused by this typo.