"More fit" only makes sense with regards to the pressure. I.e. the bacteria may survive antibiotics, but that may come at the expense of general pathogenicity.
The fact that the gene is not already widespread suggests that in the absence of the selective pressure of the antibiotic it has other effects that decrease fitness in a typical environment.
The fact that the gene is not already widespread suggests that in the absence of the selective pressure of the antibiotic it has other effects that decrease fitness in a typical environment.