The virus will mutate at a given rate. It has about 30,000 nucleotides. The animal coronaviruses in the study beginning this thread are only 20-30% homologous to SARS CoVs. In other words, about 20,000 of the 30,000 nucleotides have changed. This amount of change takes a long time.
The viruses representing the animal reservoir are 99.8% homologous. so there are only 50-60 changes. which happen in a much shorter time frame (the several months after the jump). Thus, the recent event involves viruses with a small number of changes. The recombinant event involves changes of 10's of thousands of changes.
In other words. the recombination had nothing to do with the recent jump, which involved an animal reservoir like that found in masked palm civets in Guangdong Province.