Over time, it devolved into a claim that the virus originated in experiments to enhance the infectivity of microbes being studied in the lab (so-called gain-of-function experiments) — and ultimately to the proposition that researchers at the institute unwittingly became infected while doing fieldwork and carried the virus into the institute, from which it escaped through inattention.
Different people were advocating for these different theories from the start. The thing that evolved was just the average credence for each theory. This also suggests Hiltzik did not read the book, as infection while doing fieldwork is only one of the possible vectors. Another major one is infection in the lab (which, as the authors explain, happened before at WIV with less-deadly coronaviruses).