It would be very easy to know about the toy conspiracy if they knew about the Hobi Hobi no Mi in the first place (there's a DF encyclopedia out there). Then they would just need to look at a place filled with living toys and put two and two together.
If they knew. Might be an unknown fruit. Of course, the fact it has a name might indicate it's already listed, but it's not a direct confirmation.
Someone has to give a name to a fruit that is discovered for the first time, so Soldier could have heard it from Doula goons