> Did everyone always know that solar/crop dual use was
often purely beneficial for both? (emphasis added)
"Often" != "Always". Therefore you also agree that solar still competes for land use with crops.
Furthermore, land that has already been allocated to grid scale solar has already been taken away from possible crop use. Sometimes it's not arable land, but that isn't always the case, therefore solar is still ostensibly taking up land that could be used for crops.
Finally, I even disagree that somebody knew that solar and farming could use the same land. Someone had an idea that maybe they could coexist, then ran an experiment that succeeded. They certainly didn't know the outcome beforehand.