@Danila
I'm already betting the author didn't do their homework. Although it's not just medieval Europe. Until the advent of industrial agriculture, the percentage of people who were farmers didn't drop below 70% worldwide. (With hunter-gatherers, everyone was in food production.)
@AlphaFlow
I could believe that some lords might have tried 90%, but farmers need to eat too, thermodynamically speaking. Normally in a story like this I'd have said "eh, it must be magic," but the villagers seemed shocked by how fast the MC was working, so it seems like magic isn't a replacement for industrial farming equipment in this world.