Yeah, it's like they ran out of budget to design more icons so they just kept using the yellow and red triangles.You dont need the civilopedia to understand the bonus for gunship vs. tank. The icon for the bonus is very self explanatory a yellow triangle. You know, the same icon for almost every other promotion in the game. Duh? How hard is it to figure out?
Seriously, the Civilopedia is the strongest evidence that Civ V was rushed out. It is clear that many of the game details were not yet firmed up when the Civilopedia was written, which is why the writing staff had to make do with putting down vague and unhelpful information.