They sure knew Franky and Iceburg were connected to each other and Tom during the entire flashback in Water 7, if you need to go read it again. Anyway, Wano containing an Ancient Weapon doesn’t appear to be common knowledge yet, and with all we learned in this chapter (in between the Yamato stuff), there’s no reason to think it’ll stay secret long enough for the WG to get there before it’s deployed.
Wg should know because the point of the poneglyphs was to tell the secrets that the world government kept for themselves. The WG knows the void history better than anyone