A big way this might fail is if 2020 society knows everything between them needed to use 2020 artifacts to get more 2020 artifacts, but don’t know how to use 1200 artifacts to get 2020 artifacts.
I think this is missing another important way in which this fails: even if they knew how to use 1200 artifacts to get 2020 artifacts, by moving them back in time (without also moving their stuff) you’ve made them massively poorer. If, say, you transported Elon Musk and all the relevant experts back to 1200, you might find that suddenly a much higher fraction of them are working in construction and agriculture than were previously.
I think this is missing another important way in which this fails: even if they knew how to use 1200 artifacts to get 2020 artifacts, by moving them back in time (without also moving their stuff) you’ve made them massively poorer. If, say, you transported Elon Musk and all the relevant experts back to 1200, you might find that suddenly a much higher fraction of them are working in construction and agriculture than were previously.