You should be asking yourself that question. I posted that portion for a reason, mainly due to the fact that you apparently do not do much reading, but I do understand what it means. I also understand what this means .... A variety of experiments have looked very hard for mutations at sites that are not under selection and have failed to find such mutations
The fact remains that no other mutations were found despite looking hard for them and despite any apologetic explanation for not finding them.
Spontaneous, i.e. they are not the direct planned result of some sort of directed mutation.
You left out this important notation.
The key feature that distinguishes adaptive mutations from growth-dependent mutations is that the former are specific to the selective challenge that is imposed
Plus again this....
The large contribution of IS30 insertions at the hot spot in late-arising mutants raises the question of whether that hot spot alone accounts for those highly significant differences. It does not.
Genetic variation from using error prone DNA polymerase introduces mutation throughout the genome.
That this particular screen found two mutations that restored function does not in any way imply that..
a) the mutations were anything other than spontaneous, as the authors clearly state
b) that somehow a “computing” cell knew which regions of DNA needed to be mutated and directed mutagenesis to that region.