I think it is implausible that an unrelated coronavirus with a FCS would magically make itself very genetically similar to existing sarbecoronaviruses. Of course it could make itself phenotypically similar (like whales are to sharks) but the genome would look very different.
Most very large changes is viral evolution is lateral transfer between viruses, rather than accumulation of point mutations. The better claim would be that this was acquired by a proto-SARS-CoV-2 virus that way, not that it was the result of cross-species changes alone.
I think it is implausible that an unrelated coronavirus with a FCS would magically make itself very genetically similar to existing sarbecoronaviruses. Of course it could make itself phenotypically similar (like whales are to sharks) but the genome would look very different.
https://en.wikipedia.org/wiki/RaTG13#Phylogenetics
Most very large changes is viral evolution is lateral transfer between viruses, rather than accumulation of point mutations. The better claim would be that this was acquired by a proto-SARS-CoV-2 virus that way, not that it was the result of cross-species changes alone.