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.

Besides an enormous number of people now infected with both HIV and COVID-19. That alone increases the chances of this happening naturally enormously.

my favorite part about your response here is how you didn't answer any of the questions and gave a hand-wavey response about how they're wrong.

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.