I don't find that calculation convincing as it ignores that these viruses already have a furin cleavage site that by definition must be pretty similar to the sequence here. So the probablity would have to be calculated as "how likely is it that a virus with a related furin cleavage site accumulates the number of mutations necessary to arrive at this particular optimized one". And even then as this sequence provides a fitness advantage a naive calculation could be seriously off as well.
The probability is also some argument by increduality anyway: 10-11, or you know, an occurrence rate of 10 after a trillion attempts.
In an "average" COVID-19 infection course in an adult human, it is estimated that at peak infection a person has 10^9 to 10^11 virion particles in their body alone. Multiply by the all the people infected, + all the animals, + parallel gene transfer with other viruses in the ecosystem...
