“They” can say all kinds of things but it doesn’t make them true. Corn was not available at the time of the geonim either (not that the geonim had anything to do with this gezera, which was completely unknown where they lived, but it was also not available in 12th-century France, where it seems this gezera originated) and yet everyone agrees it is forbidden.
And don’t be so sure that making flour has anything to do with the reason for the gezera. Some achronim suggested it as a possible reason, but who says they were right?