You chopped off a crucial part of the sentence that completely changes its meaning. The real sentence is:
> By aggregating VOC-associated and antibody-selected spike substitutions into a single polymutant spike protein, we show that 20 naturally occurring mutations
Aggregating ... into a polymutant spike. That means they put all 20 mutations together, each of them carefully selected from variants of concern or antibody attachment sites, into one single spike protein. Basically they created the most crazy supermutant resistent spike they could. The chance of which would be astronomically small in nature. And their conclusion is that even then a good dose of vaccine allows the body to eliminate it.
> By aggregating VOC-associated and antibody-selected spike substitutions into a single polymutant spike protein, we show that 20 naturally occurring mutations
Aggregating ... into a polymutant spike. That means they put all 20 mutations together, each of them carefully selected from variants of concern or antibody attachment sites, into one single spike protein. Basically they created the most crazy supermutant resistent spike they could. The chance of which would be astronomically small in nature. And their conclusion is that even then a good dose of vaccine allows the body to eliminate it.