The interesting thing about that theory is: I don't know how to assess which of those two scenarios is more likely.
While one might say that the same thing was happening in the market via natural gene replication and disease transmission, don't you think that someone intentionally altering parts of genes with the sole purpose of "gain-of-function" (making a more deadly virus) would be more likely to generate something like Covid-19 than a fishmonger who slaps another tilapia onto the ice?
https://duckduckgo.com/?q=gain+of+function
""Gain-of-function" is the euphemism for biological research aimed at increasing the virulence and lethality of pathogens and viruses. GoF research is government funded; its focus is on enhancing the pathogens' ability to infect different species and to increase their deadly impact as airborne pathogens and viruses.
https://duckduckgo.com/?q=gain+of+function&t=opera&ia=web&ia..."