It would be very interesting to do this same study earlier than December 2019, since quite a lot of bood from donors in that month tested positive already

Serologic testing of U.S. blood donations to identify SARS-CoV-2-reactive antibodies: December 2019-January 2020

https://academic.oup.com/cid/advance-article/doi/10.1093/cid...

So maybe the lab theory is true and it was a leak from a US lab