In the lab we can vastly exceed natural mutation rates, rendering this appeal to nature baseless.
The rate of evolution is a function of the mutation rate and the effective population size. There is no reason to believe that a lab setting, with highly parallel evolution on a very large, diverse population, will be as slow as passage through hosts in nature. The application of mutagens, and also the lack of selection by a host immune system can support much higher rates of change. And in coronaviruses, recombination is also very frequent, and this could easily give rise to multiple % levels of divergence in a single step. Finally, it's trivial to synthesize a genome of this size, and also to synthesize pools of related viruses based on common backgrounds.
The rate of evolution is a function of the mutation rate and the effective population size. There is no reason to believe that a lab setting, with highly parallel evolution on a very large, diverse population, will be as slow as passage through hosts in nature. The application of mutagens, and also the lack of selection by a host immune system can support much higher rates of change. And in coronaviruses, recombination is also very frequent, and this could easily give rise to multiple % levels of divergence in a single step. Finally, it's trivial to synthesize a genome of this size, and also to synthesize pools of related viruses based on common backgrounds.