They would've still been on Zou for quite a long time after Oden's death though, which is more than enough time to learn of the ability. Also, considering the main players of the Straw Hat Alliance went into constant detail on their game plan, it would seem very unlikely they wouldn't bring this up as a key factor. Then again, they didn't bring up the Kid Pirates, so Oda removing elements of the battle plan for the surprise factor (despite the lack of realism in doing so) wouldn't be unexpected at this point.