the mutation rate simply is not as high as you imagine it to be. If it were, then we would (likely) see more wobbling around the wobble pairs in the cleavage site coding region.
I never mentioned mutation rate so your gotcha is not as crafty as you might think. Also a mutation in the sense you seem to be implying (random base pair changes due to lack of proof reading) is not really what I am talking about. I am talking about HIV-1 genome being incorporated into SARS-CoV-2 by the host cell or either virus during replication. That is not that crazy when you have quadrillion of replication cycles and a selective pressure to mutate and incorporate new RNA.
we're talking about the odds of the founder event. My guess is odds are about 1 in 3^{5-6} (3-ish wobble options, five to six wobble sites, haven't looked at the sequence to confirm this. Probably someone can do a better analysis based on codon usage in humans. I suggest doing it as an exercise in understanding molecular biology.
alright, do you want a specific answer? My guess is it's somewhere around one in 3^{5-6}-ish, for the singular founder event that establishes and fixes that sequence as the canonical sequence for the furin cleavage site of COVID-19.